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

Abstract

We present the first results using a new technique that combines femtosecond pump-probe methods with energy-and angle-resolved photoelectron-photoion coincidence imaging. The dominant dissociative multiphoton ionization ͑DMI͒ pathway for NO 2 at 375.3 nm is identified as three-photon excitation to a repulsive potential surface correlating to NO(C 2 ⌸)ϩO( 3 P) followed by one-photon ionization to NO ϩ (X 1 ⌺ ϩ ). Dissociation along this surface is followed on a femtosecond timescale.The capability for following photodissociation dynamics directly in time is an exciting development in chemical dynamics brought about by the availability of femtosecond lasers. Initially, femtosecond time-resolved laser-induced fluorescence 1 and absorption 2 techniques were applied as probes of dissociation, but subsequently time-resolved photoionization probes, using mass spectrometry and photoelectron spectroscopy, were developed and applied to a substantially wider range of molecules and more complicated molecular systems. 1,3-5 Time-resolved mass spectrometric techniques that provide one-dimensional photofragment energy and angular resolution have also been developed for studies of reactions with multiple dissociation pathways. 6 However, when using photoionization probes, it is advantageous to detect both the photoion and photoelectron in coincidence, 7 since both particles provide important information on the dissociation event. This communication presents a new femtosecond time-resolved photoionization probe technique that combines photoelectron-photoion coincidence ͑PEPICO͒ 8,9 detection with three-dimensional energyand angle-resolved imaging. 10,11 The present studies follow the time evolution of the dissociation process that occurs during dissociative multiphoton ionization ͑DMI͒ of NO 2 induced by femtosecond laser pulses centered at 375.3 nm ͑3.30 eV͒. This technique also provides time-resolved identification of the dissociation products and ionization pathway.There have been a tremendous number of previous studies on dissociation and ionization processes in NO 2 . 12-14 Recently, there has been increased interest in the femtosecond dynamics of these processes, 15 particularly multiphoton ionization. 16,17 Other work at 375 nm ͑Ref. 16͒ found that DMI strongly dominates over simple parent ionization. This is an interesting result since parent ionization ͑9.59 eV͒ is accessible by three-photon absorption, whereas four photons are required for DMI ͑12.37 eV͒. The DMI channel was postulated to be one-photon dissociation to ground-state NOϩO ͑3.12 eV͒, followed by three-photon ionization to NO ϩ . However, the present work demonstrates that a different DMI channel dominates.A schematic diagram of the experimental setup is shown in Fig. 1. Laser pulses of 100 fs duration ͑determined from the single-shot autocorrelation width of 160 fs͒ are produced from a regeneratively amplified titanium sapphire laser system ͑Clark MXR͒, operating at 2.2 kHz. The pulses are frequency doubled to a peak wavelength of 375.3 nm using a BBO cryst...

Femtosecond Coincidence Imaging of Multichannel Multiphoton Dynamics

“…If this were a direct dissociative ionization, an inverse relationship between the electron and photofragment recoil energies would be expected [1,12]. For example, higher fragment recoil energies would correlate with lower electron energies.…”

“…So far, photoelectron-photoion coincident (PEPICO) imaging techniques have been mostly applied in ionization studies using one-photon excitation with synchrotron or He(I) radiation [6 -11]. Only recently, first femtosecond time-resolved PEPICO imaging experiments were reported [12,13], and new theoretical frameworks have been developed to study time-resolved photoelectron dynamics [14,15]. In this Letter we use the full potential of time-resolved coincidence imaging to unravel competing ionization and fragmentation processes in multiphoton excited CF 3 I.…”

“…The pump pulse (centered at 265 nm, pulse energy 13 J) and the probe pulse (centered at 398 nm, pulse energy 11 J) are focused (focal length 1 m) into an ultrahigh vacuum chamber where they intersect the pulsed molecular beam (6% CF 3 I in argon). The electrons and ions, detected in coincidence, are accelerated in opposite directions towards time-and positionsensitive detectors [12]. From the arrival time and position the initial velocity vector is calculated for each detected fragment and its associated electron.…”

“…He(I)] to probe the dissociation of theà A 2 A 1 state of CF 3 I and measure molecular-frame PADs (MF-PADs) associated with the CF 3 and I fragments [6]. Here we apply femtosecond time-resolved multiphoton excitation of CF 3 I to the region just below theà A 2 A 1 state of CF 3 I and follow the production of CF 3 I fragments using the PEPICO imaging apparatus at Sandia National Laboratories [1,12,13].…”

“…He(I)] to probe the dissociation of theà A 2 A 1 state of CF 3 I and measure molecular-frame PADs (MF-PADs) associated with the CF 3 and I fragments [6]. Here we apply femtosecond time-resolved multiphoton excitation of CF 3 I to the region just below theà A 2 A 1 state of CF 3 I and follow the production of CF 3 I fragments using the PEPICO imaging apparatus at Sandia National Laboratories [1,12,13].In the experiments, a regeneratively amplified Ti:sapphire laser operating at 0.7 kHz produces linearly polarized laser pulses at 795 nm. The compressor output, with typical pulse duration of about 100 fs, is frequency doubled and tripled.…”