Femtosecond time-resolved photoelectron–photoion coincidence imaging studies of dissociation dynamics

Imaging photoelectron photoion coincidence spectroscopy with velocity focusing electron optics

An imaging photoelectron photoion coincidence spectrometer at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source is presented and a few initial measurements are reported. Monochromatic synchrotron VUV radiation ionizes the cooled or thermal gas-phase sample. Photoelectrons are velocity focused, with better than 1 meV resolution for threshold electrons, and also act as start signal for the ion time-of-flight analysis. The ions are accelerated in a relatively low, 40-80 V cm(-1) field, which enables the direct measurement of rate constants in the 10(3)-10(7) s(-1) range. All electron and ion events are recorded in a triggerless multiple-start/multiple-stop setup, which makes it possible to carry out coincidence experiments at >100 kHz event frequencies. As examples, the threshold photoelectron spectrum of the argon dimer and the breakdown diagrams for hydrogen atom loss in room temperature methane and the chlorine atom loss in cold chlorobenzene are shown and discussed.

A photoelectron-photoion coincidence imaging apparatus for femtosecond time-resolved molecular dynamics with electron time-of-flight resolution of sigma=18 ps and energy resolution Delta E/E=3.5%

We report on the construction and performance of a novel photoelectron-photoion coincidence machine in our laboratory in Amsterdam to measure the full three-dimensional momentum distribution of correlated electrons and ions in femtosecond time-resolved molecular beam experiments. We implemented sets of open electron and ion lenses to time stretch and velocity map the charged particles. Time switched voltages are operated on the particle lenses to enable optimal electric field strengths for velocity map focusing conditions of electrons and ions separately. The position and time sensitive detectors employ microchannel plates (MCPs) in front of delay line detectors. A special effort was made to obtain the time-of-flight (TOF) of the electrons at high temporal resolution using small pore (5 microm) MCPs and implementing fast timing electronics. We measured the TOF distribution of the electrons under our typical coincidence field strengths with a temporal resolution down to sigma=18 ps. We observed that our electron coincidence detector has a timing resolution better than sigma=16 ps, which is mainly determined by the residual transit time spread of the MCPs. The typical electron energy resolution appears to be nearly laser bandwidth limited with a relative resolution of DeltaE(FWHM)/E=3.5% for electrons with kinetic energy near 2 eV. The mass resolution of the ion detector for ions measured in coincidence with electrons is about Deltam(FWHM)/m=14150. The velocity map focusing of our extended source volume of particles, due to the overlap of the molecular beam with the laser beams, results in a parent ion spot on our detector focused down to sigma=115 microm.

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

The multiphoton multichannel photodynamics of NO(2) has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and "slow" charged particle extraction using additional electron and ion optics. The NO(2) photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm. The multiphoton excitation produces both NO(2) (+) parent ions and NO(+) fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO(2) (+) or NO(+) and the (NO(+),e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the A (2)B(2) state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the NO(2) (+) parent ion. Furthermore, Rydberg states are believed to be used as "stepping" states responsible for the rather narrow and well-separated photoelectron spectra in the NO(2) (+) parent ion. Slow statistical and fast direct fragmentation of NO(2) (+) after prompt photoelectron ejection is observed leading to formation of NO(+)+O. Fragmentation from both the ground state and the electronically excited a (3)B(2) and b (3)A(2) states of NO(2) (+) is observed. At short pump probe delay times, the dominant multiphoton pathway for NO(+) formation is a 3x400 nm+1x266 nm excitation. At long delay times (>500 fs) two multiphoton pathways are observed. The dominant pathway is a 1x400 nm+2x266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3x400 nm photon absorption to NO(2) Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(A (2)Sigma,v=1) fragments, ionized by one 266 nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO(2). We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.

Dynamics on the HOCO potential energy surface studied by dissociative photodetachment of HOCO− and DOCO−

An experimental study of the dissociative photodetachment (DPD) dynamics of HOCO(-) and DOCO(-) at a photon energy of 3.21 eV has been carried out to probe the potential energy surface of the HOCO free radical and the dynamics of the OH+CO-->H+CO(2) reaction. These photoelectron-photofragment coincidence experiments allow the identification of photodetachment processes leading to the production of stable HOCO free radicals and both the H+CO(2) and OH+CO dissociation channels on the neutral surface. Isotopic substitution by deuterium in the parent ion is observed to reduce the product branching ratio for the D+CO(2) channel, consistent with tunneling playing a role in this dissociation pathway. Other isotope effects on the detailed partitioning of kinetic energy between photoelectrons and photofragments are also discussed. The results are compared to recent theoretical predictions of this DPD process, and evidence for the involvement of vibrationally excited HOCO(-) anions is discussed.

Femtosecond time-resolved photoelectron imaging

Femtosecond time-resolved photoelectron imaging (TRPEI) is a variant of time-resolved photoelectron spectroscopy used in the study of gas-phase photoinduced dynamics. A new observable, time-dependent photoionization-differential cross section provides useful information on wave-packet motions, electronic dephasing, and photoionization dynamics. This review describes fundamental issues and the most recent works involving TRPEI.

Construction and calibration of an instrument for three-dimensional ion imaging

We describe a new instrument based on a delay-line detector for imaging the complete three-dimensional velocity distribution of photoionized products from photoinitiated reactions. Doppler-free [2+1] resonantly enhanced multiphoton ionization (REMPI) of H and D atoms formed upon photolysis of HBr and DBr in the range 203 nm < or = lambda photolysis < or = 243 nm yields radial speeds measured to be accurate within 1% of those calculated. The relative speed resolution is about 5% and limited by photoionization recoil broadening. A relative speed resolution of 3.4% is obtained for [3+1] REMPI, which minimizes the ionization recoil. We also determine the branching ratio between ground-state and spin-orbit-excited product channels and their associated anisotropies. We find that DBr photolysis dynamics differs slightly from its HBr counterpart.

Dissociative photodetachment dynamics of the iodide-aniline cluster

The photodetachment dynamics of the iodide-aniline cluster, I-(C6H5NH2), were investigated using photoelectron-photofragment coincidence spectroscopy at several photon energies between 3.60 and 4.82 eV in concert with density functional theory calculations. Direct photodetachment from the solvated I- chromophore and a wavelength-independent autodetachment process were observed. Autodetachment is attributed to a charge-transfer-to-solvent reaction in which incipient continuum electrons photodetached from I- are temporarily captured by the nascent neutral iodine-aniline cluster configured in the anion geometry. Subsequent dissociation of the neutral cluster removes the stabilization, leading to autodetachment of the excess electron. The dependence of the dissociative photodetachment (DPD) and autodetachment dynamics on the final spin-orbit electronic state of the iodine fragment is characterized. The dissociation dynamics of the neutral fragments correlated with autodetached electrons were found to be identical to the DPD dynamics of the I atom product spin-orbit state closest to threshold at a given photon energy, lending support to the proposed sequential mechanism.

Femtosecond Multidimensional Imaging of a Molecular Dissociation

The coupled electronic and vibrational motions governing chemical processes are best viewed from the molecule's point of view-the molecular frame. Measurements made in the laboratory frame often conceal information because of the random orientations the molecule can take. We used a combination of time-resolved photoelectron spectroscopy, multidimensional coincidence imaging spectroscopy, and ab initio computation to trace a complete reactant-to-product pathway-the photodissociation of the nitric oxide dimer-from the molecule's point of view, on the femtosecond time scale. This method revealed an elusive photochemical process involving intermediate electronic configurations.

Time-resolved photoion and photoelectron imaging of NO2

Time-resolved photoion and photoelectron velocity mapped images from NO(2) excited close to its first dissociation limit [to NO(X(2)Pi) + O((3)P(2))] have been recorded in a two colour pump-probe experiment, using the frequency-doubled and frequency-tripled output of a regeneratively amplified titanium-sapphire laser. At least three processes are responsible for the observed transient signals; a negative pump-probe signal (corresponding to a 266 nm pump), a very short-lived transient close to the cross-correlation of the pump and probe pulses but on the 400 nm pump side, and a longer-lived positive pump-probe signal that exhibits a signature of wavepacket motion (oscillations). These transients have two main origins; multiphoton excitation of the Rydberg states of NO(2) by both 266 and 400 nm light, and electronic relaxation in the 1(2)B(2) state of NO(2), which leads to a quasi-dissociated NO(2) high in the 1(2)A(1) electronic ground state and just below the dissociation threshold. The wavepacket motion that we observe is ascribed to states exhibiting free rotation of the O atom about the NO moiety. These states, which are common for loosely bound systems such as a van der Waals complex but unusual for a chemically-bound molecule, have previously been observed in the frequency domain by optical double resonance spectroscopy but never before in the time domain.

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Dissociative Photodetachment Dynamics of Solvated Iodine Cluster Anions

Photoelectron-photofragment coincidence spectroscopy of I- (CO2), I- (NH3), I- (H2O), I- (C6H5NH2), and I- (C6H5OH) clusters was used to study the dissociative photodetachment (DPD) dynamics at 257 nm. Photodetachment from all five clusters was observed to yield bound neutral clusters as well as the DPD products of the iodine atom and the molecular solvent. Photoelectron images and kinetic energy spectra were recorded in coincidence with both the translational energy released between dissociating neutral products and stable neutral clusters. The variation of the photoelectron angular distributions in the clusters was measured, revealing significant perturbations relative to I- for I- (H2O) and I- (C6H5NH2). Product branching ratios for stable versus dissociative photodetachment and photodetachment to the I(2P(3/2)) and I(2P(1/2)) states are reported. The measurements reveal a dependence of the DPD dynamics on the final spin-orbit state of iodine in the cases of I- (C6H5NH2) and I- (CO2) and a threshold detachment process in I- (C6H5NH2).

A new approach for fast, simultaneous NO/NO2 analysis: application of basic features of multiphoton-induced ionization and dissociation of NOx

A new method of simultaneously recording NO and NO2 concentrations in complex gas mixtures is described. This method is based on resonance enhanced multiphoton ionization (REMPI), on time-of-flight mass analysis, and on monitoring the kinetic energy released upon dissociation of NO2. Its benefits are high speed and high flexibility. NO/NO2 analysis can therefore be combined with the simultaneous monitoring of other components. For instance, NH3 is a compound of interest when studying the chemical reactions of NO(x) in catalytic converters of combustion engines. The spectroscopic excitation schemes used for this new method are discussed in detail. Its reliability has been demonstrated by performing measurements at an industrial motor test facility. This novel technique performs well in comparison with conventional NO(x) analysis using chemiluminescence detection.

Dynamic molecular interferometer: Probe of inversion symmetry in I2− photodissociation

Time-resolved photoelectron imaging of negative ions is employed to examine 780-nm dissociation dynamics of I2(-), emphasizing the effects of interference in time-resolved photoelectron angular distributions obtained with 390-nm probe. No energetic changes are observed after about 700 fs, but the evolution of the photoelectron anisotropy persists for up to 2.5 ps, indicating that the electronic wave function of the dissociating anion continues to evolve long after the asymptotic energetic limit of the reaction has been effectively reached. The time scale of the anisotropy variation corresponds to a fragment separation of the same order of magnitude as the de Broglie wavelength of the emitted electrons (lambda=35 A). These findings are interpreted by considering the effect of I2(-) inversion symmetry and viewing the dissociating anion as a dynamic molecular-scale "interferometer," with the electron waves emitted from two separating centers. The predictions of the model are in agreement with the present experiment and shed new light on previously published results [A. V. Davis, R. Wester, A. E. Bragg, and D. M. Neumark, J. Chem. Phys. 118, 999 (2003)].

Mapping rotational coherences onto time-resolved photoelectron imaging observables

We explore the information content of time-resolved photoelectron imaging, a potentially powerful pump-probe technique whose popularity has been rapidly growing in recent years. To that end, we identify a mapping of the alignment properties of time-evolving wave packets onto the moments of the photoelectron images and investigate its origin and consequences theoretically and numerically.

Time-resolved imaging of the reaction coordinate

Time-resolved photoelectron imaging of negative ions is employed to study the dynamics along the reaction coordinate in the photodissociation of IBr(-). The results are discussed in a side-by-side comparison with the dissociation of I(2) (-), examined under similar experimental conditions. The I(2) (-) anion, extensively studied in the past, is used as a reference system for interpreting the IBr(-) results. The data provide rigorous dynamical tests of the anion electronic potentials. The evolution of the energetics revealed in the time-resolved (780 nm pump, 390 nm probe) I(2) (-) and IBr(-) photoelectron images is compared to the predictions of classical trajectory calculations, with the time-resolved photoelectron spectra modeled assuming a variety of neutral states accessed in the photodetachment. In light of good overall agreement of the experimental data with the theoretical predictions, the results are used to construct an experimental image of the IBr(-) dissociation potential as a function of the reaction coordinate.

Excited-state decay of hydrocarbon radicals, investigated by femtosecond time-resolved photoionization: Ethyl, propargyl, and benzyl

The excited state decay of the hydrocarbon radicals ethyl, C(2)H(5); propargyl, C(3)H(3); and benzyl, C(7)H(7) was investigated by femtosecond time-resolved photoionization. Radicals were generated by flash pyrolysis of n-propyl nitrite, propargyl bromide, and toluene, respectively. It is shown that the 2 (2)A(') (3s) Rydberg state of ethyl excited at 250 nm decays with a time constant of 20 fs. No residual signal was observed at longer delay times. For the 3 (2)B(1) state of propargyl excited at 255 nm a slower decay with a time constant 50+/-10 fs was determined. The 4 (2)B(2) state of benzyl excited at 255 nm decays within 150+/-30 fs.

Femtosecond lasers in gas phase chemistry

This critical review is intended to provide an overview of the state-of-the-art in femtosecond laser technology and recent applications in ultrafast gas phase chemical dynamics. Although "femtochemistry" is not a new subject, there have been some tremendous advances in experimental techniques during the last few years. Time-resolved photoelectron spectroscopy and ultrafast electron diffraction have enabled us to observe molecular dynamics through a wider window. Attosecond laser sources, which have so far only been exploited in atomic physics, have the potential to probe chemical dynamics on an even faster timescale and observe the motions of electrons. Huge progress in pulse shaping and pulse characterisation methodology is paving the way for exciting new advances in the field of coherent control.

Intense-field modulation of NO2 multiphoton dissociation dynamics

We report on the dynamics of multiphoton excitation and dissociation of NO(2) at wavelengths between 395 and 420 nm and intensities between 4 and 10 TW cm(-2). The breakup of the molecule is monitored by NO A (2)Sigma(+)n(')=1,0-->X (2)Pi(r)n(")=0 fluorescence as a function of time delay between the driving field and a probe field which depletes the emission. It is found that generation of n(')=0 and 1 NO A (2)Sigma(+) results in different fluorescence modulation patterns due to the intense probe field. The dissociation dynamics are interpreted in terms of nuclear motions over light-induced potentials formed by coupling of NO(2) valence and Rydberg states to the applied field. Based on this model, it is argued that the time and intensity dependences of A (2)Sigma(+)n(')=0-->X (2)Pi(r)n(")=0 fluorescence are consistent with delayed generation of NO A (2)Sigma(+)n(')=0 via a light-induced bond-hardening brought about by the transient coupling of the dressed A (2)B(2) and Rydberg 3ssigma (2)Sigma(g) (+) states of the parent molecule. The increasingly prompt decay of A (2)Sigma(+)n(')=1-->X (2)Pi(r)n(")=0 fluorescence with increasing intensity, on the other hand, is consistent with a direct surface crossing between the X (2)A(1) and 3ssigma (2)Sigma(g) (+) dressed states to generate vibrationally excited products.

Femtosecond coincidence imaging of multichannel multiphoton dynamics

The novel technique of femtosecond time-resolved photoelectron-photoion coincidence imaging is applied to unravel dissociative ionization processes in a polyatomic molecule. Femtosecond coincidence imaging of CF3I photodynamics illustrates how competing multiphoton dissociation pathways can be distinguished, which would be impossible using photoelectron or ion imaging alone. Ion-electron energy correlations and photoelectron angular distributions reveal competing processes for the channel producing (e(-)+CF+3+I). The molecular-frame photoelectron angular distributions of the two major pathways are strikingly different.

Theory of time-resolved photoelectron imaging. Comparison of a density functional with a time-dependent density functional approach

Time-resolved photoelectron differential cross sections are computed within a quantum dynamical theory that combines a formally exact solution of the nuclear dynamics with density functional theory (DFT)-based approximations of the electronic dynamics. Various observables of time-resolved photoelectron imaging techniques are computed at the Kohn-Sham and at the time-dependent DFT levels. Comparison of the results serves to assess the reliability of the former method and hence its usefulness as an economic approach for time-domain photoelectron cross section calculations, that is applicable to complex polyatomic systems. Analysis of the matrix elements that contain the electronic dynamics provides insight into a previously unexplored aspect of femtosecond-resolved photoelectron imaging.

Time-resolved electron detachment imaging of the I[sup −] channel in I[sub 2]Br[sup −] photodissociation

The evolution of the I(-) channel in I(2)Br(-) photodissociation is examined using time-resolved negative-ion photoelectron imaging spectroscopy. The 388 nm photodetachment images obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the I(-) fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a approximately 300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragment's electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, approximately 1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolved angular distributions are analyzed and found consistent with the formation of the asymptotic I(-) fragment.

Dissociative multiphoton ionization of NO[sub 2] studied by time-resolved imaging

We have studied dissociative multiphoton ionization of NO2 by time-resolved velocity map imaging in a two-color pump-probe experiment using the 400 and 266 nm harmonics of a regeneratively amplified titanium-sapphire laser. We observe that most of the ion signal appears as NO+ with approximately 0.28 eV peak kinetic energy. Approximately 600 fs period oscillations indicative of wave packet motion are also observed in the NO+ decay. We attribute the signal to two competitive mechanisms. The first involving three-photon 400 nm absorption followed by dissociative ionization of the pumped state by a subsequent 266 nm photon. The second involving one-photon 400 nm absorption to the 2B2 state of NO2 followed by two-photon dissociative ionization at 266 nm. This interpretation is derived from the observation that the total NO+ ion signal exhibits biexponential decay, 0.72 exp(-t/90+/-10)+0.28 exp(-t/4000+/-400), where t is the 266 nm delay in femtoseconds. The fast decay of the majority of the NO+ signal suggests a direct dissociation via the bending mode of the pumped state. .

Non-adiabatic intramolecular and photodissociation dynamics studied by femtosecond time-resolved photoelectron and coincidence imaging spectroscopy

Time-resolved photoelectron spectroscopy (TRPES) is emerging as a useful tool for the study of non-adiabatic dynamics in isolated polyatomic molecules and clusters due to its sensitivity to both electronic and vibrational dynamics. A powerful extension of TRPES, coincidence imaging spectroscopy (CIS), based upon femtosecond time-resolved 3D momentum vector imaging of both photoions and photoelectrons in coincidence, is a new technique for the study of complex dissociative processes. Here we show how these spectroscopies can be used to study both non-adiabatic intramolecular and photodissociation dynamics in polyatomic molecules. Intramolecular dynamics in the alpha, beta-enones acrolein, crotonaldehyde and methyl vinyl ketone are studied using both TRPES and laser-induced fluorescence of HCO(X) product yields. The location of the methyl group is seen to have very dramatic effects on the relative electronic relaxation rates and the HCO(X) yield. Applying both TRPES and CIS to the 200 nm and 209 nm photodissociation of the nitric oxide dimer, (NO)2, we observe the fs time-scale evolution of the excited parent neutral via its photoelectron spectrum and the emergence of the NO(A) photofragment including its energy and angular distributions.

Pump-probe photoionization study of the passage and bifurcation of a quantum wave packet across an avoided crossing

The application of femtosecond pump-probe photoelectron spectroscopy to directly observe vibrational wave packets passing through an avoided crossing is investigated using quantum wave packet dynamics calculations. Transfer of the vibrational wave packet between diabatic electronic surfaces, bifurcation of the wave packet, and wave packet construction via nonadiabatic mixing are shown to be observable as time-dependent splittings of peaks in the photoelectron spectra.

Photoelectron angular distributions

Angle-resolved photoelectron spectroscopy has been performed for more than 70 years in various guises, but recently its potential to help solve in detail problems in the photoionization dynamics and intramolecular dynamics of gas-phase molecules has been recognized. One key development has been the design of experiments in appropriate geometries to extract information that pertains to the molecular frame, another has been the development of imaging spectrometers, and a third is the use of ultrafast lasers to cause photoionization. In this review, which is aimed at experimentalists, simple expressions for photoelectron angular distributions (PADs) in various experimental geometries are given and their applications explained.

Femtosecond time-resolved photoelectron spectroscopy of polyatomic molecules

Femtosecond time-resolved photoelectron spectroscopy is emerging as a useful technique for investigating excited state dynamics in isolated polyatomic molecules. The sensitivity of photoelectron spectroscopy to both electronic configurations and vibrational dynamics makes it well suited to the study of ultrafast nonadiabatic processes. We review the conceptual interpretation of wavepacket dynamics experiments, emphasizing the role of the final state. We discuss the advantages of the molecular ionization continuum as the final state in polyatomic wavepacket experiments and show how the electronic structure of the continuum can be used to disentangle electronic from vibrational dynamics. We illustrate these methods with examples from diatomic wavepacket dynamics, internal conversion in polyenes and polyaromatic hydrocarbons, excited state intramolecular proton transfer, and azobenzene photoiosomerization dynamics.

Theory of time-resolved photoelectron imaging: nonperturbative calculation for an internally converting polyatomic molecule

We present the first calculation of time-resolved photoelectron angular distributions for a polyatomic system. Our method takes rotations into exact account, treats the laser field nonperturbatively, and computes the electronic dynamics from first principles. Our results point to the information content of time-resolved photoelectron imaging observables and illustrate the role played by the field intensity.

Probing the transition state via photoelectron and photodetachment spectroscopy of H(3)O(-)

The H(3)O(-) anion has stable and metastable structures that resemble configurations in the vicinity of the transition state for the neutral reactions OH + H(2) <--> H(2)O + H. Photoelectron spectroscopy of this anion probes the neutral reaction dynamics in the critical transition-state region. Accurate quantum dynamics calculations of the photoelectron intensity and photodissociation product energies are shown to provide a quantitatively reliable means of interpreting such experimental observations and reveal a detailed picture of the reaction dynamics.