Electronic and Tunneling Predissociations in the 2pπC¹Π_u^±(υ = 19) and 3pπD¹Π_u^±(υ = 4, 5) States of D₂ Studied by a Combination of XUV Laser and Velocity Map Imaging

Abstract

The predissociation mechanism of D near the threshold for the production of the D(2s, 2p) fragments has been studied by measuring the fragment yield spectra, fragment velocity map images, and fragment branching ratios D(2s)/(D(2s) + D(2p)) using a combination of XUV laser and velocity map imaging. The predissociation dynamics of the 2pπCΠ(υ = 19) and 3pπDΠ(υ = 4,5) states were studied. The 2pπCΠ(υ = 19) state is a bound state due to a shallow barrier. For the R(0) transition to the 2pπCΠ(υ = 19) state, the experimental results suggest that the predissociation occurs via three channels with decreasing importance: l-uncoupling with the 2pσBΣ state, tunneling, and l-uncoupling with the 3pσB'Σ state. For the R(1) transition to the 2pπCΠ(υ = 19) state, the first channel plays the dominant role. For the Q(1) transition to the 2pπCΠ(υ = 19) state, the predissociation occurs via tunneling as required by symmetry. For the predissociation of the 3pπDΠ(υ = 4,5) states, the experimental data confirm the earlier results indicating that the main perturbing state is 3pσB'Σ. The Beutler-Fano profiles and the associated spectroscopic parameters for the various predissociations have also been obtained. The measured Fano-parameters q for the P- and R-branches of the 3pπDΠ state are found to have opposite signs, and their relationships are in agreement with a formula derived from the Fano equation. Rotationally resolved Beutler-Fano profiles were measured for the P(2) and P(3) lines.

Spectra and Predissociation Dynamics of the 4pσB′′1∑u+ and 4pπD′ 1Πu States of D2

“…Therefore, higher energy states of the 2pπC state can dissociate via tunneling or coupling with the continuum of the 2pσB state, and both of them correlate to the H­(1s) + H­(2p) channels. The branching ratio measurements showed that the products were indeed mainly of the H­(2p) state . For the predissociation of the 3pπD 1 Π u + state, the angular distribution of the fragments and the channel branching ratio [H­(2p)]/[H­(2p) + H­(2s)] showed that dissociating state has 1 ∑ u + symmetry correlating to H­(1s) + H­(2s), which confirms that the predissociation occurs via the coupling with the vibrational continuum of the 3pσB′ 1 ∑ u + state. − …”

“…Our experimental setup has been described in previous publications, − and only a summary is given here. The setup consists of a tunable XUV pump laser (83.19–83.68 nm), a probe laser at 364 nm, and a typical supersonic molecular beam machine equipped with a velocity map imaging component. , The XUV laser was generated by four-wave sum frequency mixing with one fixed frequency laser, one tunable laser, and one pulsed Kr jet as the nonlinear medium.…”

“…The Rydberg states of H 2 have been studied using a variety of experimental and theoretical tools since the early days of quantum mechanics. − Recently, using a synchrotron radiation light source and Fourier transform techniques, the Rydberg spectra of subwavenumber resolutions and absolute energy accuracies of a few hundredths of wavenumber have been obtained, ranging from the lowest Rydberg state of principal quantum number n = 2 to the Rydberg states up to n = 69. − To study the decay dynamics of the Rydberg states, the spectra of XUV light-induced molecular fluorescence, molecular ionization, and Lyman-α emission of the H/D-fragments have been recorded. − High-resolution spectra, 0.3 to 0.02 cm –1, , based on XUV laser generated by four-wave-mixing method, have also been recorded for photon energies 14–15 eV. − Theoretical calculations using multichannel quantum defect theories provided accurate energy levels, absorption intensities, and line widths comparable to experimental data. − , …”

“…The fragment state distribution and angular distribution, which are closely related to the predissociation mechanism, have been paid much less attention. Recently, we have performed such measurements by using the XUV laser pump and UV laser probe method. − The angular distributions of the fragments were measured using the velocity map imaging method, and the dissociation channel branching ratios between the H­(2s) and H­(2p) states were measured using the so-called delay-time-curve method. Our studies focused on the 2pπC and 3pπD states.…”

Quantum state-to-state vacuum ultraviolet photodissociation dynamics of small molecules

“…These changes are resulted from the competitions between the direct dissociation channels and the predissociation channels at different positions within the Beutler-Fano profile. For D 2 , very similar ex-perimental measurements have been performed for the 2pπC 1 Π ± u (v=19) and 3pπD 1 Π ± u (v=4, 5) states [20]. Besides the normal electronic predissociation process, tunneling predissociation also plays an important role for the 2pπC 1 Π ± u (v=19) state, especially for the f-parity levels for which tunneling is the only predissociation channel as required by the symmetry.…”