C.‐L. Liao

Vacuum ultraviolet photodissociation and photoionization studies of CH3SH and SH

The kinetic energy releases of the photodissociation processes, CH3SH+hν (193 nm)→CH3+SH, CH3S+H, and CH2S+H2, have been measured using the time‐of‐flight mass spectrometric method. These measurements allow the direct determination of the dissociation energies for the CH3–SH and CH3S–H bonds at 0 K as 72.4±1.5 and 90±2 kcal/mol, respectively. The further dissociation of SH according to the process SH+hν (193 nm)→S+H has also been observed. The appearance energy (AE) of S produced in the latter process is consistent with the formation of S(3P)+H. The photoelectron–photoion coincidence (PEPICO) spectra for CH3SH+, CH3S+ (or CH2SH+), and CH2S+ from CH3SH have been measured in the wavelength range of 925–1460 A. The PEPICO measurements make possible the construction of the breakdown diagram for the unimolecular decomposition of internal‐energy‐selected CH3SH+ in the range of 0–83 kcal/mol. The AE measured for CH2S+ is consistent with the conclusion that the activation energy is negligible for 1,2‐H2 eliminati...

Vacuum ultraviolet photodissociation and photoionization studies of CH3SCH3 and CH3S

We have measured the translational energy releases of the laser photodissociation processes CH3SCH3+hν (193 nm)→CH3+CH3S [process (1)] and CH3SCH2+H [process (2)]; and CH3S+hν (193 nm)→S+CH3 [process (3)]. The onsets of the translational energy distributions for photofragments of processes (1) and (2) allow the direct determination of 74.9±1.5 and 91±2.5 kcal/mol for the dissociation energies of the CH3–SCH3 and H–CH2SCH3 bonds at 0 K, respectively. The threshold observed for S formed by process (3) is consistent with the conclusion that the production of S(3P) is small compared to S(1D). The photoelectron–photoion coincidence (PEPICO) spectra for CH3SCH+3, CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ resulting from photoionization of CH3SCH3 have been measured in the wavelength region of 900–1475 A. The PEPICO study allows the construction of a detailed breakdown diagram for the formation of CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ from energy‐selected CH3SCH+3 ions.

A state‐to‐state study of the electron transfer reactions Ar+(2P3/2,1/2)+N2(X̃,v=0)→Ar(1S0) +N+2(X̃,v’)

The vibrational state distributions of N+2(X,v’) ions resulting from the reactions, Ar+(2P3/2)+N2(X,v=0)→Ar(1S0) +N+2(X,v’) [reaction (1)] and Ar+(2P1/2)+N2(X,v=0)→Ar(1S0) +N+2(X,v’) [reaction (2)], over the center‐of‐mass collisional energy (Ec.m.) range of 0.25–41.2 eV in a crossed ion–neutral beam experiment have been probed by the charge exchange method. The experimental results obtained for reaction (1) are in accord with the predictions of the semiclassical multistate calculation of Spalburg and Gislason that N+2 ions are formed predominantly (≳85%) in the v’=1 state and that the production of N+2(X,v’=0) becomes more important as Ec.m. is increased. The experiment also supports the theoretical results for reaction (2) at Ec.m.=1.2 and 4.1 eV showing that ≳80% of N+2 product ions are in the v’=2 state. However, the calculation is found to either over‐estimate the populations for N+2(v’ 2) resulting from reaction (2) at Ec.m.=10.3and 41.2 eV. Absol...

A 193 nm laser photofragmentation time‐of‐flight mass spectrometric study of CH3SSCH3, SSCH3, and SCH3

We have measured the time‐of‐flight (TOF) spectra for SCH3, CH3, and SSCH3 formed in the photodissociation processes, CH3SSCH3+hν(193 nm)→2SCH3 and CH3+SSCH3. The dissociation energies for the CH3S–SCH3 and CH3SS–CH3 bonds determined at 0 K by the TOF measurements are 72.4±1.5 and 55.0±1.5 kcal/mol, in agreement with the literature values. The threshold value for the formation of S2 measured by the TOF spectrum for S2 is in accord with the thermochemical threshold for the process, SSCH3+hν(193 nm) →S2+CH3. The threshold energy determined from the TOF spectrum for S is found to be consistent with the thermochemical threshold for the photodissociation process, SCH3+hν(193 nm) →S(1D)+CH3, an observation supporting that S atoms are not produced in the ground S(3P) state in the 193 nm photodissociation of SCH3. This observation is rationalized by symmetry correlation arguments applied between the S+CH3 product and SCH3 states.

Theoretical and experimental total state-selected and state-to-state cross sections. III, The (Ar+H2)+ system

A detailed three‐dimensional quantum mechanical study of the (Ar+H2)+ system along the energy range 0.4 eV≤Etot≤1.65 eV is presented. The main difference between this new treatment and the previously published one [J. Chem. Phys. 87, 465 (1987)] is the employment of a new version of the reactive infinite‐order sudden approximation (IOSA), which is based on the ordinary inelastic IOSA carried out for an optical potential. In the numerical treatment we include three surfaces (only two were included in the previous treatment), one which correlates with the Ar+H+2 system and two which correlate with the two spin states of Ar+(2Pj); j=3/2,1/2. The results are compared with both trajectory‐surface‐hopping calculations and with experiments. In most cases, very good agreement is obtained.

Experimental and theoretical studies of isomeric C2H5S and C2H5S

Abstract By combining photoionization and photodissociation measurements with ab initio Gaussian-2 (G2) calculations on the C 2 H 5 S and C 2 H 5 S + system, we have concluded that CH 3 CH 2 S is the dominant primary product formed in the 193 nm photodissociation of (CH 3 CH 2 ) 2 S, while CH 3 CHSH + is the product ion formed at the photoionization onset of C 2 H 5 S + from (CH 3 CH 2 ) 2 S. The G2 predictions for the heats of formation at 0 K (Δ H f0 ) for the isomers ofC 2 H 5 S and C 2 H 5 S + (CH 3 CH 2 S:Δ H f0 (G2)=27.5 kcal/mol;CH 3 SCH 2 :Δ f0 (G2)=37.3kcal/mol; CH 3 CH 2 S + :Δ H f0 (G2)=236.5 kcal/mol;CH 3 CHSH + :Δ H f0 (G2)=192.6 kcal/mol; and CH 3 SCH 2 + : Δ H f0 (G2)=195.3 kcal/mol) are in agreement with available experimental Δ H f0 values (CH 3 CH 2 S: Δ H f0 (exp)=31.4±2 kcal/mol; CH 3 SCH 2 : Δ H f0 /(exp)=34.8±2.5 kcal/mol; CH 3 CH 2 S + : Δ H f0 (exp)=238.3±2 kcal/mol; CH 3 CHSH + :Δ H f0 (exp)=189.6±1.0 kcal/mol;and CH 3 SCH 2 + :Δ H f0 (exp)=195.1 kcal/mol).The G2 calculation also yields a value of 35.3 kcal/mol for Δ H f0 (CH 3 CHSH).

Experimental and theoretical total state‐selected and state‐to‐state absolute cross sections. II. The Ar+(2P3/2,1/2)+H2 reaction

Total state‐selected and state‐to‐state absolute cross sections for the reactions Ar+(2P3/2,1/2)+H2(X,v=0)→Ar (1S0)+H+2(X,v’) [reaction (1)], ArH++H [reaction (2)], and H++H+Ar [reaction (3)] have been measured in the center‐of‐mass collision energy Ec.m. range of 0.24–19.1 eV. Absolute spin–orbit state transition total cross sections (σ3/2→1/2,σ1/2→3/2) for the collisions of Ar+(2P3/2,1/2) with H2 at Ec.m.=1.2–19.1 eV have been obtained.The measured state‐selected cross sections for reaction (1) [σ3/2,1/2(H+2)] reveal that at Ec.m.≤5 eV, σ1/2(H+2) is greater than σ3/2(H+2), while the reverse is observed at Ec.m.≥7 eV. The total state‐to‐state absolute cross sections for reaction (1) (σ3/2,1/2→v’) show unambiguously that in the Ec.m. range of 0.16–3.9 eV the dominant product channel formed in the reaction of Ar+(2P1/2)+H2(X,v=0) is H+2(X,v’=2)+Ar. These observations support the conclusion that at low Ec.m. the outcome of charge transfer collisions is governed mostly by the close energy resonance effect....

A study of the S(3P2,1,0;1D2) production in the 193 nm photodissociation of CH3S(X̃)

The dynamics of S(3P2,1,0;1D2) production from the 193 nm photodissociation of CH3SCH3 has been studied using 2+1 resonance‐enhanced multiphoton ionization techniques. The 193 nm photodissociation cross section for the formation of S from CH3S initially prepared in the photodissociation of CH3SCH3 is estimated to be 1×10−18 cm2. The branching ratio for S(3P)/S(1D) is found to be 0.15/0.85. The fine‐structure distribution observed for product S(3P2,1,0) is nearly statistical. Possible potential energy surfaces involved in the 193 nm photodissociation of CH3S(X) have been examined theoretically along the CH3–S dissociation coordinate in C3v symmetry. These calculations suggest that predissociation of CH3S(C 2A2) via the repulsive CH3S(E 2E) surface is most likely responsible for the efficient production of S(1D). For vibrationally excited CH3S(X), a viable mechanism for the dominant production of S(1D) may involve direct dissociation via the CH3S(E 2E) state formed in the 193 nm photoexcitation.

A state‐to‐state study of the symmetric charge transfer reaction Ar+(2P3/2,1/2)+Ar(1S0)

The relative state‐to‐state total charge transfer cross sections, σ3/2→3/2, σ3/2→1/2, σ1/2→1/2, and σ1/2→3/2, for the reactions Ar+(2P3/2)+Ar(1S0)  → Ar(1S0)+Ar+(2P3/2)  → Ar(1S0)+Ar+(2P1/2), Ar+(2P1/2)+Ar(1S0)  → Ar(1S0)+Ar+(2P1/2)  → Ar(1S0)+Ar+(2P3/2), respectively, at the laboratory collision energy range of 1–4000 eV, have been determined using the newly constructed crossed ion–neutral beamphotoionization apparatus. This apparatus is equipped with a high resolution photoionization ion source for reactant state selections and a charge transfer detector for product state identifications. The measured profile of the kinetic energy dependence for the probability for 2P3/2→2P1/2 fine‐structure transitions in Ar+(2P3/2)+Ar(1S0) charge transfer collisions [σ3/2→1/2/(σ3/2→3/2+σ3/2→1/2)] is in general agreement with the theoretical prediction of Johnson. However, the theoretical probabilities are approximately 40% greater than those observed in this experiment. The total charge transfer cross section for Ar+(...

Adiabatic ionization energy and electron affinity of CH2Br

The photoionization efficiency spectrum of supersonically cooled CH2Br has been measured near its ionization threshold. The adiabatic ionization energy (IE) of CH2Br is determined to be 8.61±0.01 eV, in excellent agreement with the value obtained previously using the He i photoelectron spectroscopic method. We have also performed Gaussian‐2 (G2) calculations on CH2Br+, CH2Br, and CH2Br− which yield values of 8.47 and 0.97 eV for the IE and electron affinity of CH2Br, respectively. The G2 electron affinity is in accord with the literature value of 1.0±0.3 eV calculated from the acidity of CH3Br.