Rovibrational photoionization dynamics of methyl and its isotopomers studied by high-resolution photoionization and photoelectron spectroscopy

Abstract

High-resolution photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3), CH(2)D, CHD(2), and CD(3) have been recorded in the vicinity of the first adiabatic ionization threshold following single-photon excitation from the ground neutral state using a narrow-bandwidth vacuum-ultraviolet laser. The radicals were produced from the precursor molecules methyl-bromide, methyl-iodide, dimethyl-thioether, acetone, and nitromethane by 193 nm excimer photolysis in a quartz capillary and were subsequently cooled to a rotational temperature T(rot) approximately equal to 30 K in a supersonic expansion. Nitromethane was identified as a particularly suitable photolytic precursor of methyl for studies by photoionization and threshold photoelectron spectroscopy. Thanks to the cold rotational temperature reached in the supersonic expansion, the rotational structure of the threshold ionization spectra could be resolved, and the photoionization dynamics investigated. Rydberg series converging on excited rotational levels of CH(3) (+) could be observed in the range of principal quantum number n=30-50, and both rotational autoionization and predissociation were identified as decay processes in the threshold region. The observed photoionization transitions can be understood in the realm of an orbital model for direct ionization but the intensity distributions can only be fully accounted for if the rotational channel interactions mediated by the quadrupole of the cation are considered. Improved values of the adiabatic ionization thresholds were derived for all isotopomers [CH(3): 79 356.2(15) cm(-1), CH(2)D: 79 338.8(15) cm(-1), CHD(2): 79 319.1(15) cm(-1), and CD(3): 79 296.4(15) cm(-1)].

Vacuum-UV negative photoion spectroscopy of CH3F, CH3Cl and CH3Br

“…The AE values of X − formation presented in Table 1 and Figure 4 are used with the IE of the CH 3 radical (9.84 ± 0.01 eV) 36 and the EA of the respective halogen atom (F (3.401 eV); Cl (3.613 eV); Br (3.364 eV)) 37 for the C−X bond cleavage, and with the IE of H Table 3, and are compared to literature values. 41 An alternative way to present the data for XH 2 C−H bond cleavage is to use literature values for the bond dissociation energies, and calculate a lower limit to the electron affinity of the CH 2 X radical.…”

Single-photon and resonance-enhanced multiphoton threshold ionization of the allyl radical

“…Because of the absence of sharp resonances in the photoionization spectrum which results, at least in part, from spectral congestion, the intensity distribution observed in the photoelectron spectrum is not expected to be strongly perturbed by the effects of channel interactions, which often complicate the interpretation of PFI-ZEKE photoelectron spectra. 25,34 The PFI-ZEKE photoelectron spectra of both bands are congested with little rotational structure resolved apart from a reproducible doublepeak structure close to the band center with a separation of about 9 cm rotational temperatures of 25 K ͓Fig. 3͑a͔͒ and 35 K ͓Fig.…”

“…Tunable vacuumultraviolet ͑VUV͒ radiation in the region of the adiabatic ionization threshold was generated by resonance-enhanced difference-frequency mixing ͑ VUV =2 1 filled with krypton using the ͑4p͒ 5 ͑ 2 P 3/2 ͒5p͓1 / 2͔͑J =0͒ ← ͑4p͒ 6 1 S 0 two-photon resonance in krypton at 2 1 = 94 092.862 cm −1 as described in Ref. 25. Photoionization spectra were measured by recording the C 3 H 5 + ion signal mass selectively as a function of the VUV wave number VUV .…”

Rovibronic photoionization dynamics of ammonia isotopomers

“…Moreover, different rotational selection rules are obtained for NH 2 D and ND 2 H because the different orientations of the inertial axes imply a different symmetry classification of the rotational wavefunctions. A completely analogous dependence of the rotational selection rules on the isotopic substitution was recently observed in the photoionization of the methyl radical CH 3 and its mixed isotopomers CH 2 D and CD 2 H [32].…”

“…Additional photoionization propensity rules can be derived within the framework of the rotation-spectator/ orbital-ionization model [7,8,21,22,[32][33]. A detailed discussion of the model for the photoionization of asymmetric-top molecules is given in [22,34].…”