Bastian Noller

Threshold photoelectron spectroscopy of the methyl radical isotopomers, CH3, CH2D, CHD2 and CD3: synergy between VUV synchrotron radiation experiments and explicitly correlated coupled cluster calculations.

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.

Photoionization of Propargyl and Bromopropargyl Radicals: A Threshold Photoelectron Spectroscopic Study

In this Article, we present mass-selected threshold photoelectron spectra of propargyl as well as the 1- and 3-bromopropargyl radicals. The reactive intermediates were produced by flash pyrolysis of suitable precursors and ionized by VUV synchrotron radiation. The TPES of the propargyl radical was simulated using data from a recent high-level computational study. An ionization energy (IE) of 8.71 ± 0.02 eV was obtained, in excellent agreement with computations, but slightly above previous experimental IEs. The pyrolysis of 1,3-dibromopropyne delivers both 1- and 3-bromopropargyl radicals that can be distinguished by their different ionization energies (8.34 and 8.16 eV). To explain the vibrational structure, a Franck-Condon simulation was performed, based on DFT calculations, which can account for all major spectral features. Bromopropargyl photoionizes dissociatively beginning at around 10.1 eV. Cationic excited states of 1- and 3-bromopropargyl were tentatively identified. The dissociative photoionization of the precursor (1,3-dibromopropyne) was also examined, delivering an AE(0K) (C(3)H(2)Br(+)/C(3)H(2)Br(2)) of 10.6 eV.

Photodissociation dynamics of the 2-propyl radical, C3H7

The photodissociation of 2-propyl leading to propene+H was investigated with nanosecond time resolution. A supersonic beam of isolated 2-propyl radicals was produced by pyrolysis of 2-bromopopane. The kinetic energy release of the H-atom photofragment was monitored as a function of excitation wavelength by photofragment Doppler spectroscopy via the Lyman-alpha transition. The loss of hydrogen atoms after excitation proceeds in alpha position to the radical center with a rate constant of 5.8x10(7) s-1 at 254 nm. Approximately 20% of the excess energy is deposited as translation in the H-atom photofragment. In contrast 1-propyl does not lose H atoms to a significant extent. The experimental results are compared to simple Rice-Ramsperger-Kassel-Marcus calculations. The possible reaction pathways are examined in hybrid density functional theory calculations.

Threshold Photoelectron Spectroscopy of Cyclopropenylidene, Chlorocyclopropenylidene, and Their Deuterated Isotopomeres

Cyclopropenylidene (c-C(3)H(2)), chlorocyclopropenylidene (c-C(3)HCl), and their deuterated isotopomers were studied by the threshold photoelectron-photoion coincidence (TPEPICO) technique using VUV synchrotron radiation. The carbenes were generated via flash pyrolysis. In all species a change in geometry is visible upon ionization, with significant activity in the C═C, C-C-stretching mode and, in the case of c-C(3)H(2)/D(2), the C-H-bending mode. The electron is removed from an sp(2) like hybrid orbital centered on the carbene C atom. The mass selected threshold photoelectron (TPE) spectra were fitted by a Franck-Condon simulation, yielding the equilibrium geometry of the cation ground state ((1)A(1)). The adiabatic ionization energy IE(ad) of c-C(3)H(2) was determined to be 9.17 eV, in good agreement with calculations and literature values. Two vibrational wavenumbers of the cation were determined experimentally (ν(3)(+) = 1150 cm(-1) and ν(2)(+) = 1530 cm(-1)). Chlorocyclopropenylidene was also studied by TPE spectroscopy and has a similar IE(ad) of 9.17 eV. The spectrum also shows a vibrational progression that corresponds to the C═C- and C-C-stretching modes of the cation. The equilibrium geometry was also determined by a Franck-Condon fit. The IE(ad) of the deuterated isotopomers, c-C(3)D(2) and c-C(3)DCl, were also determined to be 9.17 eV. The spectra confirm the assignments for the nondeuterated species.

On the photodissociation of propadienylidene, l-C3H2

We investigate the photochemistry and photodissociation dynamics of the linear C3H2 isomer propadienylidene by two-colour photofragment Doppler spectroscopy at excitation wavelengths between 260 and 230 nm, corresponding to excitation into the C1 A1 state. Propadienylidene is generated by pyrolysis from IC3H2Br. Almost complete conversion of the precursor can only be achieved at high pyrolysis temperatures. Two reaction channels, H-atom loss and loss of H2 molecules, are energetically close. Our results show that H-atom loss is indeed important in the investigated energy range. The Doppler profiles indicate that 34-37% of the excess energy is released as translation. The rates for H-atom loss are faster than the time resolution of our nanosecond-laser setup, kH > 10(8) s(-1).

Time- and frequency-resolved photoionization of the C A22 state of the benzyl radical, C7H7

The structure and dynamics of the C (2)A(2) electronically excited state of the benzyl radical, C(7)H(7), were investigated by nanosecond and femtosecond pump-probe photoionization. A free jet of benzyl radicals was generated by flash pyrolysis from the precursors 2-phenylethyl nitrite and toluene. Nanosecond multiphoton ionization spectra show a number of vibronic bands that are excited in the wavelength range of 290-310 nm. At excitation wavelengths of 305, 301, and 298 nm, rapid biexponential decay of the excited states was observed. Lifetimes at the C-state origin (305 nm excitation) are 400 fs and 4.5 ps. The lifetimes decrease with increasing excitation energy. The dynamics can be understood within a two-step internal conversion to the electronic ground state.

Side-Chain Effects on the Electronic Relaxation of Radicals followed by Time-Resolved Pump—Probe Spectroscopy: 2,3-Dimethylbut-2-yl vs tert-Butyl

The excited-state lifetime of the 2,3-dimethylbut-2-yl (DMB) radical, a hexyl isomer, upon electronic excitation into the 3p Rydberg state at 265 nm, is measured by femtosecond time-resolved photoionization. It is shown that the 3p state deactivates in a two-step process, which is well described by two time constants of 25 and 400 fs. The results are compared to tert-butyl (t-C(4)H(9)), another tertiary radical investigated before. Time-dependent DFT calculations confirm the earlier suggestion that curve crossings along the C-C coordinate play an important role in the excited-state deactivation.