Translational spectroscopy of H(D) atom fragments arising from the photodissociation of H₂S(D₂S): a redetermination of D00(S–H)

Abstract

The technique of H/D atom photofragment translational spectroscopy has been used to further investigate the collision-free photodissociation of H,S and D,S molecules both in the near ultraviolet (at 218.2 and 221.6 nm) and in the vacuum ultraviolet (at 121.6 nm). Measurements of the H/D atom photofragment angular distributions confirms that the near UV dissociation occurs promptly, following a perpendicular photo-excitation. More than 99% of the resulting SH/SD fragments are formed in their ground vibronic level, with a ca. 3 : 2 preference in favour of the lower (2113/2) spin-orbit component. Product rotation accounts for ca. 1% of the available energy in the case of H2S photolysis at these near UV wavelengths (ca. 2% in the case of 0,s dissociation). The groundstate SH/SD photofragments can also be photolysed at these near UV excitation wavelengths. Simulations of the kinetic energy distribution of the resulting H/D atomic fragments show that the secondary photolysis also involves a perpendicular transition, and that the partner S atoms are formed in all three 3P, s p i w r b i t states. The product energy disposal following 121.6 nm photolysis of D2S closely parallels that deduced in an earlier study of H,S photodissociation at this same wavelength (Schnieder eta!., J. Chern. Phys., 1990, 92, 7027). The D-atom kinetic energy spectrum shows clear evidence for the formation of rovibrationally excited SD(A 'C+) fragments amongst the primary products, and also suggests an important role for the three-body dissociation process leading to D + D + S('D) atoms. 40 cm-', the present results provide a refined value for the S-D bond strength in the D,S molecule; Dg(DS-D) = 32030 & 50 cm-' ; for the SH and SO radical bond dissociation energies: Dg(S-H) = 29300 & 100 cm-' and Dg(S-0) = 29700 & 100 cm-', and an improved expression for the potential-energy function for the A 'C+ state of the mercapto radical.

Near ultraviolet photolysis of phosphine studied by H atom photofragment translational spectroscopy

“…Such a suggestion is not entirely original (40,42). It is also reminiscent of the photochemistry reported recently for the SH/SD(X) fragments resulting from near UV photodissociation of H2S and D2S (45,46).…”

“…Dissociations proceeding via [12] are thus presumed to contribute to the broad underlying maximum centred at Ekin -5000 cm-'. (Such a model of the secondary dissociation process is reminiscent of that occurring in H2S and D2S following excitation to t h e E '~, surface (31,45).) The solid curves in Fig.…”

Two-photon resonance enhanced MPI-PES above the lowest ionization threshold. Observation of the [a1Δ]5pπ 2Φ state of the SH (SD) radical

“…Constants for the zero point level of the X 211 ground state of SH [1] and SD [8] and the [a 1A ]4p'rr 20 state of SD [3] are included for comparison. tion of the population over the ground state rotational levels as determined in previous studies on the photodissociation of H~S (D2S) [11,12], i.e. the overall population in the ~ H 3/2 and 2171/2 spin-orbit states of SH (SD) is in the ratio of 3:2, and the rotational distribution over each of these states is characterized by a Boltzmann temperature of 300 K.…”

Two-photon resonance enhanced multiphoton ionization photoelectron spectroscopy of the SH (SD) radical below and above the lowest ionization threshold

“…25 The photofragmentation of H 2 S ͑D 2 S͒ has been the subject of a large number of investigations in which the spin-orbit, vibrational, and rotational state distributions of the SH ͑SD͒ fragments in their ground state have been measured for several photodissociation wavelengths. [26][27][28][29][30][31][32][33] It has been shown that after photodissociation of H 2 S ͑D 2 S͒, using various near-ultraviolet photodissociation wavelengths, SH ͑SD͒ fragments are formed in their ground vibronic level with a ϳ3:2 ratio over the 2 ⌸ 3/2 and 2 ⌸ 1/2 spin-orbit components, and with a rotational distribution over each of these components which can be char- Values obtained in the present study are given in Table III. acterized by a Boltzmann temperature of ϳ300 K. In the present study H 2 S ͑D 2 S͒ photodissociation and SH ͑SD͒ twophoton resonance enhanced multiphoton ionization are carried out with identical near-ultraviolet excitation wavelengths between 258 and 208 nm. Employing the above approach we have been able to investigate 14 Rydberg states of SH ͑SD͒ which had not been observed before.…”

“…In simulating the spectra we assumed an isotropic molecular sample whose overall population in the 2 ⌸ 3/2 and 2 ⌸ 1/2 spin-orbit states of SH ͑SD͒ is in the ratio 3:2, and whose rotational distribution over each of these states is characterized by a Boltzmann temperature of 300 K. 32 Deviations from the above distribution over In a ͑2ϩ1͒ REMPI process the intensity of a MPI transition is proportional to the product of the two-photon cross section for excitation to the excited state level from the ground state, and the one-photon ionization cross section from the upper level. In this multiphoton ionization process deviations from the expected intensities can arise from interference effects resulting from a near resonance at the onephoton level as well as from interaction between states at the resonant two-photon level.…”