Photodissociation of S₂ (X³Σ_g^–, a¹Δ_g, and b¹Σ_g⁺) in the 320–205 nm Region

Abstract

Photodissociation of vibrationally and electronically excited sulfur dimer molecules (S 2 ) has been studied in a combined experimental and computational quantum chemistry study in order to characterize bound-continuum transitions. Ab initio quantum chemistry calculations are carried out to predict the potential energy curves, spin–orbit coupling, transition moments, and bound-continuum spectra of S 2 for comparison with the experimental data. The experiment uses velocity map imaging to measure S-atom production following S 2 photoexcitation in the ultraviolet region (320–205 nm). A pulsed electric discharge in H 2 S produces ground-state S 2 X 3 Σ g – ( v = 0–15) as well as electronically excited singlet sulfur and b 1 Σ g + ( v = 0, 1), and evidence is presented for the production and photodissociation of S 2 a 1 Δ g . In a previous paper, we reported threshold photodissociation of S 2 X 3 Σ g – ( v = 0) in the 282–266 nm region. In the present study, S( 3 P J ) fine structure branching and angular distributions for photodissociation of S 2 (X 3 Σ g – ( v = 0), a 1 Δ g and b 1 Σ g + ) via the B″ 3 Π u , B 3 Σ u – and 1 1 Π u excited states are reported. In addition, photodissociation of the X 3 Σ g – ( v = 0) state of S 2 to the second dissociation limit producing S( 3 P 2 ) + S( 1 D) is characterized. The present results on S 2 photodynamics are compared to those of the well-studied electronically isovalent O 2 molecule.

Multiphoton Ionization/Dissociation of Molecular Sulfur S2 in the UV Region

“…All signals corresponded to one-photon dissociation to DL 0 ; no signals for two-photon dissociation (to 3 P products) or to photodissociation of S 2 + ions were observed. In our other study of the metastable state S 2 (a 1 Δ g , b 1 Σ g + ), around 310 nm 25 onephoton dissociation is also the stronger channel compared to two-photon dissociation processes. At the S( 1 S) and S( 1 D) REMPI wavelengths longer than 280 nm, two-photon dissociation was found to be stronger than three-photon dissociation to S* or dissociation of S 2 + .…”

“…In the UV wavelength region, images were recorded under tight focusing conditions. In our previous two-color study, we reported images taken with the dissociation laser in the 281–266 nm region at low laser intensity and the detection laser tightly focused for S( 3 P J ) detection around 308 nm . With tight focusing in the region below 288 nm, strong S + images were found at or near the wavelengths corresponding to the X ( 3 Σ g – ) – B ( 3 Σ u – )( v ′) bands shown schematically in the energy level diagram of Figure and in Figure .…”

“…In our previous two-color study, we reported images taken with the dissociation laser in the 281− 266 nm region at low laser intensity and the detection laser tightly focused for S( 3 P J ) detection around 308 nm. 25 With tight focusing in the region below 288 nm, strong S + images were found at or near the wavelengths corresponding to the + electronic states and dissociation limits DL 0−5 to neutral "valence" atoms and to excited Rydberg states (in which case the partner is an S( 3 P 2 ) atom). Vertical black arrows indicate the energy of a 280 nm photon, which is just above the DL 0 at the one-photon level.…”

“…Later, we investigated the photodissociation of singlet sulfur S 2 a 1 Δ g and b 1 Σ g + (i.e., the metastable singlet states of S 2 ) in a single laser experiment. 25 The main goal of this work is to extend the study of photodissociation of S 2 at multiphoton levels in order to explore various dynamical processes in S 2 .…”

“…Frederix et al studied the photodissociation of S 2 at a one-photon level with subsequent detection of S­( 3 P j ) fragments using VMI where the bond energy of S 2 was significantly revised to a value of 4.418 eV. Later, we investigated the photodissociation of singlet sulfur S 2 a 1 Δ g and b 1 Σ g + (i.e., the metastable singlet states of S 2 ) in a single laser experiment . The main goal of this work is to extend the study of photodissociation of S 2 at multiphoton levels in order to explore various dynamical processes in S 2 .…”