Laser probing of the electron-impact excited c(2p)³ _umanifold of states in H₂

Abstract

A stepwise laser excitation method has been used to probe individual ro-vibrational levels in the molecular hydrogen metastable c(2p) 3Πu manifold of states. Metastable states produced by electron-impact excitation are subsequently excited by the absorption of a single UV laser photon, producing a complex triplet nd Rydberg spectrum observed via field ionization and autoionization. The spectrum has been analysed to determine quantum numbers of the states associated with the transitions. Excitation functions for individual ro-vibrational states in the c(2p) 3Πu manifold are then determined as a function of electron-impact energy.

Spectrum of the Autoionizing Triplet Gerade Rydberg States of H2 and its Analysis Using Multichannel Quantum-Defect Theory

“…There are two reasons why the intensity of the transitions decreases with increasing vibrational excitation: First, the higher vibrational levels of the metastable state are less populated in the discharge because the Franck−Condon factor for the excitation from the vibronic ground state of H 2 decreases. 43 Second, the lifetime of the metastable state decreases with vibrational excitation, 54 leading to a partial decay of the population of metastable H 2 in higher vibrational states during the ∼100 μs flight time from the discharge region to the laserexcitation region (Figure 2). Overall, the line widths of the transitions on the low-n side of the spectrum are larger than those on the high-n side.…”

“…Shifts toward lower wavenumbers of the transitions occurring at increasing υ + = υ″ values originate from the reduced vibrational spacing of H 2 + compared to the vibrational spacing of the metastable state. There are two reasons why the intensity of the transitions decreases with increasing vibrational excitation: First, the higher vibrational levels of the metastable state are less populated in the discharge because the Franck–Condon factor for the excitation from the vibronic ground state of H 2 decreases . Second, the lifetime of the metastable state decreases with vibrational excitation, leading to a partial decay of the population of metastable H 2 in higher vibrational states during the ∼100 μs flight time from the discharge region to the laser-excitation region (Figure ).…”

“…Although states with n ≤ 4 were extensively studied (see refs , , and − and also ref for a recompilation of some of the experimental data), we are not aware of any systematic analysis of triplet gerade Rydberg states with n > 4. Several groups studied high- n d triplet Rydberg states from the metastable c 3 Π u – state; − however, their spectra remained only partially analyzed, and neither vibrational perturbers nor n s Rydberg states could be definitely assigned. Dinu et al showed that, although the dominant decay channel for the triplet n d Rydberg states with υ + ≤ 4 lying below the adiabatic ionization threshold is predissociation, states above this threshold decay predominantly by autoionization.…”

Spectrum of Field-Ionized Triplet Gerade Rydberg States of H2 and Comparison to Multichannel Quantum-Defect Theory

“…Molecular hydrogen prepared in the metastable c 3 Π u – state has been used as an initial state to further investigate the triplet system with experiments on both low n ( n ≤ 4) − and high n ( n > 4). ,− A schematic diagram of the observed Rydberg states and their relation to neighboring autoionizing Rydberg series observed in other experiments is shown in Figure .…”

Fluorescence decay processes following resonant 2p photoexcitation of Ar atoms and clusters studied using a time-resolved fluorescence and photoion coincidence technique

“…Pioneering PIFCO work has been done by the Leach group [8][9][10]. Later, to identify the excited states of neutral fragments, spectra in the visible and UV region were recorded for atoms [7,11,12]. The detection of VUV fluorescence has also been used for studying dissociative molecular states [13][14][15][16][17].…”