Coherent Oscillatory Femtosecond Dynamics in Multichannel Photodynamics of NO₂ Studied by Spatially Masked Electron Imaging

Abstract

The femtosecond multiphoton photoionization and dissociation dynamics of NO(2) have been studied in a two-color pump-probe experiment at 400 and 266 nm using velocity map ion imaging in conjunction with photoelectron imaging. We report here a series of experiments focusing on the oscillatory patterns in pump-probe transients of the photoelectron signal. By using the technique of spatially masked imaging detection, we can select different photoelectron channels enabling the rapid measurement of energy selected transients with good signal-to-noise ratio. At short delay times (<300 fs) the dominant process is dissociative multiphoton ionization by 3 x 400 nm + 1 x 266 nm excitation to a repulsive potential energy surface of the NO(2)(+) cation correlating to NO(+)((1)Sigma(+)) + O((3) P) and the ejection of a 0.37 eV electron. At longer delay times (>400 fs), the release of high-energy electrons (0.88 eV) is observed attributed to a three-photon absorption at 400 nm to Rydberg and valence type excited states of neutral NO(2) leading to predissociation and the production of NO(+) + O((3)P) from a one-photon ionization at 266 nm. At longer delay times (>400 fs) a second slow (near 0 eV) photoelectron channel is observed that is associated with one photon excitation at 400 nm to the first excited A(2)B(2) state of NO(2) followed by two-photon excitation at 266 nm leading to near threshold ionization and dissociation to NO(+) + O((3)P). Distinctive oscillatory patterns were found in the pump-probe transients of the photoelectron yield for both the slow and the fast photoelectron channels but with different periods of about 750 fs (slow) or 590 fs (fast). Extensive polarization experiments are reported for both linear and circular polarized pump and probe laser geometries. We discuss the oscillatory mechanism in relation to ab initio calculations of relevant Rydberg and valence type excited states of NO(2) near 9.3 eV. We propose that an oscillating wavepacket of mixed Rydberg and valence character that predissociates is responsible for the observed oscillations in the transients of the fast (0.88 eV) photoelectron channel.

Toward elucidating the mechanism of femtosecond pulse shaping control in photodynamics of molecules by velocity map photoelectron and ion imaging

“…[35][36][37][38] In brief terms, the molecular beam was prepared by seeding 5% CH 2 BrCl in He which expands through a pulsed ͑1 kHz͒ homebuilt piezovalve with a 300 m nozzle. The molecular beam is doubly skimmed and crossed by femtosecond laser pulses about 150 mm downstream from the nozzle orifice.…”

The Reaction Microscope: Imaging and Pulse Shaping Control in Photodynamics

“…[81] The time scale of the oscillations and the polarization experiments exclude rotational wavepackets in NO 2 or NO as the origin of the oscillations. New ab initio calculations show that there are avoided crossings or conical intersections between bound and dissociative excited states of different Rydberg and valence character near 9 eV.…”

“…In a different (noncoincidence) imaging spectrometer we have recently studied in extensive polarization experiments and much more detail the distinctive oscillatory patterns (not shown here). [81] The time scale of the oscillations and the polarization experiments exclude rotational wavepackets in NO 2 or NO as the origin of the oscillations. New ab initio calculations show that there are avoided crossings or conical intersections between bound and dissociative excited states of different Rydberg and valence character near 9 eV.…”

“…Specifically, in a series of velocity map imaging experiments using femtosecond pump-probe spectroscopy at 400 nm and 266 nm, an oscillatory component in the pumpprobe transient of NO + fragments and electrons was observed. [79][80][81][82] The first extensive study performed with the novel coincidence imaging machine described above was the time-resolved dynamics in NO 2 in a two-color pump (400 nm) -probe (267 nm) scheme with laser pulses of about 120 fs duration. [65] Time-dependent photoelectron images in coincidence with NO 2 + or NO + were measured and the (NO + ,e) photoelectron versus fragment ion kinetic energy correlations were reported.…”

“…It was proposed that an oscillating and predissociating wavepacket of mixed Rydberg and valence character is responsible for the observed oscillations in the transients of the fast (0.88 eV) photoelectron. [81] Recently, Whitaker, Blanchet and coworkers further discussed NO 2 imaging experiments and the origin of the oscillations. [82] The energy correlation plots reveal the complex multiphoton multichannel pathways in the ultrafast photodynamics of a small molecule like NO 2 .…”