Photoselection and the Appearance of Franck−Condon-Forbidden Thresholds in the ZEKE Spectrum of NO₂

Abstract

Photoionizing transitions that change NO2 +−core vibrational quantum numbers are examined by means of zero-kinetic-energy (ZEKE) threshold photoelectron spectroscopy. Observable intensity is found in Franck−Condon-off-diagonal transitions from the (000) vibrational ground state of the 3pσ 2Σu + Rydberg state of NO2 to long-lived resonances at thresholds corresponding to (010), (0200), and (100) vibrationally excited states of NO2 +. Analyses of the rovibrational structure of the originating Rydberg state and the cation establish that NO2 +−core potential energy surfaces are in each case substantially parallel and thus predict zero Franck−Condon factors for transitions that change vibrational quantum numbers. The appearance of nonvertical transitions is attributed to the mixing of long-lived high-principal-quantum-number discrete Rydberg states that converge to each excited threshold with the optically accessible ground-state free-electron continuum. The vibrationally inelastic interaction that gives rise to this Franck−Condon-forbidden threshold structure mirrors the common one in which discrete Rydberg states converging to a vibrationally excited threshold decay by vibrational autoionization to an underlying relaxed-core continuum. In the present case, continuum states reached in vertical excitation form long-lived resonances by borrowing lifetime from discrete high-n states converging to vibrationally excited thresholds. Apparently, the discrete character conveyed in each case is sufficient to permit evolution in the Stark manifold that stabilizes ZEKE states for detection by pulsed-field ionization. This is the first example of a continuum stabilized by the ZEKE Stark-mixing mechanism. The extent of the effect observed at each threshold is consistent with patterns of electron−core coupling strengths established by mode-dependent trends in vibrational autoionization linewidths.

Electron impact cross sections of vibrationally and electronically excited molecules

“…That is possible only if the lifetimes of super-excited states are much higher. 270 b) Similar results were obtained in experiments carried out using zero kinetic energy (ZEKE) photoelectron spectroscopy [31, 32,33] and mass-analyzed threshold ionization (MATI) spectroscopy [34]. These experiments 272 have reported on the dynamics of the high Rydberg molecular states, which are located close to the ionization potential (IP) and have a well-defined ion cores.…”

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

“…Baltzer et al 17 observed the onset of the ionic ground state near 10.3 eV and the center of the band near 11.2 eV, substantially higher than the adiabatic energy of 9.586 eV. 31,32 Grant and co-workers [14][15][16] have used multicolor double-resonant multiphoton excitation with tunable nanosecond lasers to study Rydberg states in NO 2 and the state-selective production of vibrationally excited NO 2 + . This double-resonant scheme makes it possible to prepare low lying vibronic states in the 3p and 3p Rydberg states of NO 2 .…”

“…This is exactly in the region of vibrationally excited levels of the 3p 2 ⌸ u Rydberg state. 14,15 Further absorption of a second 266.7 nm photon from this Rydberg state will produce NO 2 + ions in coincidence with electrons with a rather narrow kinetic energy distribution due to the similarity of the Rydberg state with the ground state of the NO 2 + ion. This width may be mainly determined by the total energy spread of the multiphoton excitation due to the short femtosecond pulses.…”

“…[14][15][16][17][18][19] The femtosecond time-resolved photoelectron-photoion coincidence imaging technique was pioneered by Hayden and co-workers and applied to NO 2 , 20,21 CF 3 I, 22 and the NO dimer. 23 In particular, Hayden and co-workers studied the femtosecond dynamics of NO 2 in a single color pump-probe experiment at 375 nm.…”

High-Rydberg fragment formation via core dissociation of superexcited Rydberg molecules

“…Since the potential energy curves for the neutral and ionic states are very similar, Franck-Condon factors for such transitions are expected to be small. Yet, recent studies 3,17 have shown that such transitions could be quite significant. ͓Indirect evidence for the population of such superexcited HR states was also reported in electron attachment to molecules excited to energies above their lowest IPs ͑see Refs.…”