Este trabalho foi financiado pelo Instituto de pesquisa exército dos EUA através do contrato W911NF-18-1-0051.

<ol>
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Este protocolo permite-nos resolver ultra rápida dinâmica vibracional em polyatomic radicais cátions através de preparação seletiva dos íons no estado eletrônico de chão. Enquanto o procedimento padrão campo forte ionização usando 800 nm pode preparar vibracionais coerências em cações radical de estado eletrônico-chão de primeira-linha diatómicos10,11,12,13 e CO 2 <...

Desde a invenção do laser em 1960, o objetivo de quebrar seletivamente ligações químicas em moléculas tem sido um sonho de longa data de químicos e físicos. A capacidade de sintonizar os dois laser de frequência e intensidade foi acreditada para permitir direta clivagem de uma ligação de destino por meio de absorbância de energia seletiva nos frequência vibracional associada1,2,3,4 . No entanto, primeiras experiências descobriu que intramolecular redistribuição vibracional da energia absorvida por toda a molécula muitas vezes resultou no decote não-seletivo do vínculo mais fraco4,5. Não foi até o desenvolvimento de femtossegundo pulsado lasers e a bomba-sonda técnica6 no final dos anos 1980 que dirigem a manipulação de Estados vibracionais coerentes, ou "pacotes de onda", habilitado controle bem sucedido sobre clivagem de ligação e outros objectivos de7,6,8. Medições de bomba-sonda, no qual o pulso de "bomba" prepara um estado excitado ou íon que posteriormente é animado por um pulso de atraso "sonda", continuam a ser uma das técnicas mais amplamente utilizadas para estudar processos ultra rápidos em moléculas9, 10,11,12,13,14,15,16,17,18, 19,20.
Uma limitação significativa para estudar a dissociação ultra rápida dinâmica de polyatomic cátions radicais usando a excitação de bomba-sonda acoplada a espectrometria de massa detecção surge da fragmentação não seletivo da molécula alvo pela bomba ionizante pulso no comprimento de onda de Ti:Sapphire de 800 nm21,22,23. Essa fragmentação excessiva resulta da ionização do multiphoton nonadiabatic e podem ser atenuada por mudando o comprimento de onda de excitação para o infravermelho próximo (ex., 1200-1500 nm)22,23,24, 25. Nestes comprimentos de onda mais longos, a contribuição dos aumentos de encapsulamento adiabática elétron em relação à excitação do multiphoton no processo de ionização22,23. Tunelamento adiabática transmite pouca energia em excesso para a molécula e formas predominantemente "frio" estado fundamental íons molecular19,22,23. Nosso trabalho anterior demonstrou que o uso de excitação de infravermelho próximo melhora significativamente a preparação coerentes excitações vibracionais, ou "pacotes de onda", em polyatomic cações radicais em comparação a 800 nm excitação19, 20. Este trabalho irá ilustrar a diferença entre ionização forte-campo dominada por contribuições do multiphoton e encapsulamento com medições de bomba-sonda sobre o guerra química agente simulador dimetil metilfosfonato (DMMP) usando a 1500 nm e 800 nm comprimentos de onda da bomba.
Em nossos experimentos de bomba-sonda, um par de pulsos de laser ultracurtos é temporizada, recombinadas e focalizadas em um espectrômetro de massa de tempo-de-voo, conforme mostrado em nossa configuração na Figura 1. Estas experiências exigem um Ti:Sapphire regenerativo amplificador produzindo > 2 mJ, 800 nm, 30 fs de impulsos. A saída do amplificador é dividida em um divisor de feixe de 90:10 (% r %T), onde a maioria da energia é usada para bombear um amplificador paramétrico óptico (OPA) para geração de 1200-1600 nm, 100-300 µ j, pulsos de 20-30 fs. O diâmetro do feixe de bomba do IR é expandido para 22 mm e o diâmetro do feixe sonda 800 nm para baixo-colimado de 5,5 mm e endocarpo usando uma íris. Estes collimations resultam no feixe de bomba com foco para uma significativamente menor feixe cintura (9 µm) do que o raio da sonda (30 µm), garantindo assim que todos os íons formados durante o pulso de bomba ionizantes são excitados pelo pulso de atraso sonda. Essa configuração é usada porque o objetivo das nossas experiências é sondar a dinâmica do íon molecular pai, que pode ser formado ainda em intensidades baixas perto das bordas do feixe focalizado. Podemos constatar que se a dinâmica de mais altamente animado espécie iônica de interesse, então o diâmetro do feixe de sonda deve ser feito menor do que a bomba.
Os pulsos de bomba e sonda propagam collinearly e concentram-se para a região de extração de um espectrômetro de massa de tempo-de-voo (TOF-MS) Wiley-McLaren26 (Figura 2). Amostras moleculares, colocadas em um frasco são anexadas para a entrada e abriu para o vácuo. Esta configuração requer que a molécula sob investigação tenha uma pressão de vapor diferente de zero; para moléculas com baixa pressão de vapor, o frasco pode ser aquecido. O fluxo da amostra gasosa na câmara é controlado por duas válvulas de escape variável. A amostra incorpora a câmara através de um 1/16" inox tubo aproximadamente 1 cm longe do foco do laser (Figura 2) a fim de entregar localmente alta concentração da molécula alvo na extração região27. A placa de extração tem uma fenda de 0,5 mm orientada ortogonal para os caminhos de propagação e íon do laser. Porque o intervalo de Rayleigh do raio da bomba é de aproximadamente 2 mm, esta fenda serve como um filtro, permitindo que apenas os íons gerados a partir do volume focal central onde a intensidade é mais elevada para passar através da extração de placa28. Os íons entram um tubo de deriva de campo livre de 1 m para chegar a Z-abertura canal micro placa (MCP) detector29, onde são detectados e gravados com um osciloscópio digital de 1 GHz, à taxa de repetição de 1KHz de lasers de Ti:Sapphire comercial típica.

<table><tbody><tr><td>&lt;strong&gt;Mass spectrometer components&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>TOF lens stack and flight tube assembly</td><td>Jordan TOF Products, Inc.</td><td>C-677</td><td></td></tr><tr><td>18 mm Z-gap detector assembly</td><td>Jordan TOF Products, Inc.</td><td>C-701Z</td><td></td></tr><tr><td>TOF high voltage power supply</td><td>Jordan TOF Products, Inc.</td><td>D-603</td><td></td></tr><tr><td>&lt;strong&gt;Vacuum system components&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Rotary vane backing pump</td><td>Edwards Vacuum LLC</td><td>RV12</td><td></td></tr><tr><td>Turbomolecular pumps (2)</td><td>Edwards Vacuum LLC</td><td>EXT255H</td><td></td></tr><tr><td>Turbomolecular pump controllers (2)</td><td>Edwards Vacuum LLC</td><td>EXC300</td><td></td></tr><tr><td>Pressure gauge</td><td>Edwards Vacuum LLC</td><td>AIGX-S-DN40CF</td><td></td></tr><tr><td>Chiller for water cooling</td><td>Neslab</td><td>CFT-25</td><td></td></tr><tr><td>&lt;strong&gt;Femtosecond laser system&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Ti:Sapphire regenerative amplifier</td><td>Coherent, Inc.</td><td>Astrella</td><td>oscillator and amplifier in a single integrated system</td></tr><tr><td>Optical Parametric Amplifer (OPA)</td><td>Light Conversion</td><td>TOPAS Prime</td><td></td></tr><tr><td>&lt;strong&gt;Motion control&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Motorized linear translation stage 1&quot; travel</td><td>Thorlabs</td><td>Z825B</td><td></td></tr><tr><td>controller for linear translation stage</td><td>Thorlabs</td><td>KDC 101</td><td></td></tr><tr><td>USB controller hub and power supply</td><td>Thorlabs</td><td>KCH 601</td><td></td></tr><tr><td>Manual linear translation stage 1&quot; travel</td><td>Thorlabs</td><td>PT1</td><td></td></tr><tr><td>&lt;strong&gt;Detectors&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Pyroelectric laser energy meter</td><td>Coherent, Inc.</td><td>1168337</td><td></td></tr><tr><td>Thermal laser power meter</td><td>Coherent, Inc.</td><td>5356E16R</td><td></td></tr><tr><td>Si-biased detector 200-1100 nm</td><td>Thorlabs</td><td>DET10A</td><td></td></tr><tr><td>Compact USB CMOS Camera</td><td>Thorlabs</td><td>DCC1545M</td><td></td></tr><tr><td>USB spectrometer</td><td>Ocean Optics</td><td>HR4000</td><td></td></tr><tr><td>1 GHz digital oscilloscope&amp;nbsp;</td><td>LeCroy</td><td>WaveRunner 610Zi</td><td></td></tr><tr><td>&lt;strong&gt;Optics&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Type 1 BBO crystal</td><td>Crylight Photonics</td><td>BBO007</td><td>aperture and thickness may be customized</td></tr><tr><td>Achromatic half wave plate, 1100-2000 nm</td><td>Thorlabs</td><td>AHWP05M-1600</td><td></td></tr><tr><td>Wollaston prism polarizer</td><td>Thorlabs</td><td>WPM10</td><td></td></tr><tr><td>Hollow retro-reflector</td><td>PLX, Inc.</td><td>OW-20-1C</td><td></td></tr><tr><td>Variable neutral density filter</td><td>Thorlabs</td><td>NDC-100C-2</td><td></td></tr><tr><td>Longpass dichroic mirror 2&quot; diameter</td><td>Thorlabs</td><td>DMLP950L</td><td></td></tr><tr><td>&lt;strong&gt;Software&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Digital Camera image software</td><td>Thorlabs</td><td>ThorCam</td><td></td></tr><tr><td>Instrument communication interface</td><td>National Instruments</td><td>NI-MAX</td><td></td></tr><tr><td>Graphical development environment for measurement programs</td><td>National Instruments</td><td>LabVIEW</td><td></td></tr><tr><td>Data processing software</td><td>Mathworks</td><td>MATLAB</td><td></td></tr></tbody></table>

measurement of ultrafast vibrational coherences in polyatomic radical cations with strong-field adiabatic ionization

Apresentamos um método de bomba-sonda para preparar vibracionais coerências em polyatomic radicais cátions e sondar sua dinâmica ultra rápida. Deslocando-se o comprimento de onda do pulso forte-campo ionizante bomba do comumente usados 800 nm para o infravermelho próximo (1200-1600 nm), a contribuição do elétron adiabática de encapsulamento para o processo de ionização aumenta em relação à absorção do multiphoton. Adiabática ionização resulta em uma população predominante do estado eletrônico fundamental do íon após a remoção do elétron, que efetivamente prepara um coerente estado vibracional ("pacote de onda") favorável à excitação subsequente. Em nossos experimentos, a dinâmica coerente vibracional é analisada com pulso fraco-campo 800 nm e rendimentos de produtos de dissociação dependente do tempo medido em um espectrômetro de massa de tempo-de-voo. Apresentamos as medições sobre a molécula dimetil metilfosfonato (DMMP) para ilustrar como usar pulsos de 1500 nm para a excitação aumenta a amplitude das oscilações coerentes em rendimentos de íon por um fator de 10 em comparação com pulsos de 800 nm. Este protocolo pode ser implementado em configurações existentes de bomba-sonda através da incorporação de uma óptica amplificador paramétrico (OPA) para conversão de comprimento de onda.

São apresentados os resultados obtidos para a molécula DMMP21 . A Figura 3 mostra os espectros de massa DMMP tomados no zero tempo de atraso com as intensidades de pico da bomba 1500 nm e 800 pulsos de sonda nm sendo 8 x 1013 e 8 x 1012 W cm-2, respectivamente. Para referência, o espectro de massa tirado com apenas o pulso da bomba também é mostrado. Os espectros são em média mais de 10.000 laser...

Watch this Scientific Journal Video about Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization at JoVE.com

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

Apresentamos um protocolo para sondar ultra rápidas coerências vibracionais em polyatomic cações radicais que resultam em dissociação molecular.

Medição de coerências vibracionais ultrarrápidas em cátions de radicais poliatômicos com ionização adiabática de campo forte

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the ...

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6.8K visualizações.  Virginia Commonwealth University. Apresentamos um protocolo para sondar ultra rápidas coerências vibracionais em polyatomic cações radicais que resultam em dissociação molecular.

Vídeo: Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet (VUV) Synchrotron Radiation

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy and dynamics1-4. Fundamental studies of photoionization processes of biomolecules provide information about the electronic structure of these systems. Furthermore determinations of ionization energies and other properties of biomolecules in the gas phase are not trivial, and these experiments provide a platform to generate these data. We have developed a thermal vaporization technique coupled with supersonic molecular beams that provides a gentle way to transport these species into the gas phase. Judicious combination of source gas and temperature allows for formation of dimers and higher clusters of the DNA bases. The focus of this particular work is on the effects of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic interactions, on the ionization energies and proton transfer of individual biomolecules, their complexes and upon micro-hydration by water1, 5-9.
We have performed experimental and theoretical characterization of the photoionization dynamics of gas-phase uracil and 1,3-dimethyluracil dimers using molecular beams coupled with synchrotron radiation at the Chemical Dynamics Beamline10 located at the Advanced Light Source and the experimental details are visualized here. This allowed us to observe the proton transfer in 1,3-dimethyluracil dimers, a system with pi stacking geometry and with no hydrogen bonds1. Molecular beams provide a very convenient and efficient way to isolate the sample of interest from environmental perturbations which in return allows accurate comparison with electronic structure calculations11, 12. By tuning the photon energy from the synchrotron, a photoionization efficiency (PIE) curve can be plotted which informs us about the cationic electronic states. These values can then be compared to theoretical models and calculations and in turn, explain in detail the electronic structure and dynamics of the investigated species 1, 3.

Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet VUV Synchrotron Radiation

A molecular beam coupled to tunable vacuum ultraviolet photoionization mass spectrometer at a synchrotron provides a convenient tool to explore the electronic structure of isolated gas phase molecules and clusters. Proton transfer mechanisms in DNA base dimers were elucidated with this technique.

Espectrometria de Massa Molecular Beam Com Tunable radiação ultravioleta de vácuo Síncrotron (VUV)

Tunable soft ionization coupled to mass spectroscopy is a powerful method to investigate isolated molecules, complexes and clusters and their spectroscopy ...

Engenharia

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 &#181;m spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO).
The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation&#160;and are analyzed to infer electron density and temperature.

Medição e Análise de hidrogênio atômico e Diatomic AlO Molecular, C<sub&gt; 2</sub&gt;, CN, e TiO Spectra Following induzida por laser Breakdown Optical

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

Proton Transfer and Protein Conformation Dynamics in Photosensitive Proteins by Time-resolved Step-scan Fourier-transform Infrared Spectroscopy

Monitoring the dynamics of protonation and protein backbone conformation changes during the function of a protein is an essential step towards understanding its mechanism. Protonation and conformational changes affect the vibration pattern of amino acid side chains and of the peptide bond, respectively, both of which can be probed by infrared (IR) difference spectroscopy. For proteins whose function can be repetitively and reproducibly triggered by light, it is possible to obtain infrared difference spectra with (sub)microsecond resolution over a broad spectral range using the step-scan Fourier transform infrared technique. With ~102-103 repetitions of the photoreaction, the minimum number to complete a scan at reasonable spectral resolution and bandwidth, the noise level in the absorption difference spectra can be as low as ~10-4, sufficient to follow the kinetics of protonation changes from a single amino acid. Lower noise levels can be accomplished by more data averaging and/or mathematical processing. The amount of protein required for optimal results is between 5-100 &#181;g, depending on the sampling technique used. Regarding additional requirements, the protein needs to be first concentrated in a low ionic strength buffer and then dried to form a film. The protein film is hydrated prior to the experiment, either with little droplets of water or under controlled atmospheric humidity. The attained hydration level (g of water / g of protein) is gauged from an IR absorption spectrum. To showcase the technique, we studied the photocycle of the light-driven proton-pump bacteriorhodopsin in its native purple membrane environment, and of the light-gated ion channel channelrhodopsin-2 solubilized in detergent.

Key steps of protein function, in particular backbone conformational changes and proton transfer reactions, often take place in the microsecond to millisecond time scale. These dynamical processes can be studied by time-resolved step-scan Fourier-transform infrared spectroscopy, in particular for proteins whose function is triggered by light.

Transferência de prótons e Proteína Conformação Dynamics em fotossensíveis Proteínas por Time-resolvido Passo-scan-Fourier Transform Infrared Spectroscopy

Monitoring the dynamics of protonation and protein backbone conformation changes during the function of a protein is an essential step towards ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

Direct Imaging da Laser-driven Ultrafast rotação Molecular

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Química

Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector.
Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.

For small chiral species, Coulomb Explosion Imaging provides a new approach to determine the handedness of individual molecules.

Imagens de Explosão de Coulomb como uma Ferramenta para Distinguir Estereoisômeros

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish ...

Measurements of Long-range Electronic Correlations During Femtosecond Diffraction Experiments Performed on Nanocrystals of Buckminsterfullerene

The precise details of the interaction of intense X-ray pulses with matter are a topic of intense interest to researchers attempting to interpret the results of femtosecond X-ray free electron laser (XFEL) experiments. An increasing number of experimental observations have shown that although nuclear motion can be negligible, given a short enough incident pulse duration, electronic motion cannot be ignored. The current and widely accepted models assume that although electrons undergo dynamics driven by interaction with the pulse, their motion could largely be considered &#39;random&#39;. This would then allow the supposedly incoherent contribution from the electronic motion to be treated as a continuous background signal and thus ignored. The original aim of our experiment was to precisely measure the change in intensity of individual Bragg peaks, due to X-ray induced electronic damage in a model system, crystalline C60. Contrary to this expectation, we observed that at the highest X-ray intensities, the electron dynamics in C60 were in fact highly correlated, and over sufficiently long distances that the positions of the Bragg reflections are significantly altered. This paper describes in detail the methods and protocols used for these experiments, which were conducted both at the Linac Coherent Light Source (LCLS) and the Australian Synchrotron (AS) as well as the crystallographic approaches used to analyse the data.

We describe an experiment designed to probe the electronic damage induced in nanocrystals of Buckminsterfullerene (C60) by intense, femtosecond pulses of X-rays. The experiment found that, surprisingly, rather than being stochastic, the X-ray induced electron dynamics in C60 are highly correlated, extending over hundreds of unit cells within the crystals1.

Medições de correlações eletrônicas de longo alcance durante experimentos de difração de femtossegundo realizados em nanocristais de Buckminsterfulereno

The precise details of the interaction of intense X-ray pulses with matter are a topic of intense interest to researchers attempting to interpret the ...

An Experimental Protocol for Femtosecond NIR/UV - XUV Pump-Probe Experiments with Free-Electron Lasers

This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a free-electron laser. This includes methods to establish the spatial and temporal overlap between the optical and free-electron laser pulses during the experiment, as well as important aspects of the data analysis, such as corrections for arrival time jitter, which are necessary to obtain high-quality pump-probe data sets with the best possible temporal resolution. These methods are demonstrated for an exemplary experiment performed at the FLASH (Free-electron LASer Hamburg) free-electron laser in order to study ultrafast photochemistry in gas-phase molecules by means of velocity map ion imaging. However, most of the strategies are also applicable to similar pump-probe experiments using other targets or other experimental techniques.

This protocol describes the key steps for performing and analyzing pump-probe experiments combining a femtosecond optical laser with a free-electron laser in order to study ultrafast photochemical reactions in gas-phase molecules.

Um protocolo Experimental para Femtosecond NIR/UV - XUV bomba-sonda experimentos com Lasers de elétrons livres

This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a ...

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound electrons with neutral molecules/atoms. State-of-the-art in vacuo anion generation techniques allow application to a broad range of atomic, molecular, and cluster anion systems. These are separated and selected using time-of-flight mass spectrometry. Electrons are removed by linearly polarized photons (photo detachment) using table-top laser sources which provide ready access to excitation energies from the infra-red to the near ultraviolet. Detecting the photoelectrons with a velocity mapped imaging lens and position sensitive detector means that, in principle, every photoelectron reaches the detector and the detection efficiency is uniform for all kinetic energies. Photoelectron spectra extracted from the images via mathematical reconstruction using an inverse Abel transformation reveal details of the anion internal energy state distribution and the resultant neutral energy states. At low electron kinetic energy, typical resolution is sufficient to reveal energy level differences on the order of a few millielectron-volts, i.e., different vibrational levels for molecular species or spin-orbit splitting in atoms. Photoelectron angular distributions extracted from the inverse Abel transformation represent the signatures of the bound electron orbital, allowing more detailed probing of electronic structure. The spectra and angular distributions also encode details of the interactions between the outgoing electron and the residual neutral species subsequent to excitation. The technique is illustrated by the application to an atomic anion (F&#8722;), but it can also be applied to the measurement of molecular anion spectroscopy, the study of low lying anion resonances (as an alternative to scattering experiments) and femtosecond (fs) time resolved studies of the dynamic evolution of anions.

Here, we present a protocol for photoelectron imaging of anionic species. Anions generated in vacuo and separated by mass spectrometry are probed using velocity mapped photoelectron imaging, providing details of anion and neutral energy levels, anion and neutral structure and the nature of the anion electronic state.

Imagem de fotoelétron de ânions ilustrado por 310 Nm destacamento de F−

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional &#960;-conjugate Systems

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near infrared (near-IR) transitions, because it can overcome the low sensitivity of spontaneous Raman spectrometers in the near-IR region. Here, we describe technical details of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer that we have recently developed. A description of signal generation and optimization, measurement, data acquisition, and calibration and correction of recorded data is provided as well. We present an application of our spectrometer to analyze the excited-state dynamics of &#946;-carotene in toluene solution. A C=C stretch band of &#946;-carotene in the second lowest excited singlet (S2) state and the lowest excited singlet (S1) state is clearly observed in the recorded time-resolved stimulated Raman spectra. The femtosecond time-resolved near-IR stimulated Raman spectrometer is applicable to the structural dynamics of &#960;-conjugate systems from simple molecules to complex materials.

Details of signal generation and optimization, measurement, data acquisition, and data handling for a femtosecond time-resolved near-IR stimulated Raman spectrometer are described. A near infrared stimulated Raman study on the excited-state dynamics of &#946;-carotene in toluene is shown as a representative application.

Mediçãos de Raman estimuladas do tempo ultrarrápido de sistemas funcionais de πconjugados

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near ...

Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active metal-oxide surfaces in contact with light-absorbing dyes is an ongoing thorny and elusive challenge in physical chemistry. With this in mind, a molecular-dynamics (MD) simulation were performed by using optimized empirical potentials for a well-representative and prototypical dye-sensitized solar cell (DSC) solvated by a widely-studied room temperature ionic liquid (RTIL), in the guise of a [bmim]+[NTf2]- RTIL solvating an N719-sensitizing dye adsorbed onto 101 anatase-titania. In doing so, important insights were gleaned into how using a RTIL as the electrolytic hole acceptor modulates the dynamical and vibrational properties of a N719 dye, estimating the spectra for the DSC photo-active interface via Fourier transformation of mass-weighted velocity autocorrelation functions from MD. The acquired vibrational spectra were compared with the experiment spectra and those sampled from ab initio molecular dynamics (AIMD); in particular, various empirical-potential spectra generated from MD provide insight into how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction. In any event, careful fitting of empirical force-field models has been shown to be an effective tool in handling DSC vibrational properties, when validated by AIMD and an experiment.

A dye-sensitized solar cell was solvated by RTILs; using optimized empirical potentials, a molecular dynamics simulation was applied to compute vibrational properties. The obtained vibrational spectra were compared with experiment and ab initio molecular dynamics; various empirical potential spectra show how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction.

Espectro vibracional de uma interface N719-Cromosforre/Titania da Simulação Empírica-Potencial de Dinâmica Molecular, solvated por um líquido iônico de temperatura ambiente

Medição de coerências vibracionais ultrarrápidas em cátions de radicais poliatômicos com ionização adiabática de campo forte

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