Ingo Fischer

Femtosecond wave‐packet dynamics studied by time‐resolved zero‐kinetic energy photoelectron spectroscopy

Femtosecond pump–probe zero‐kinetic‐energy (ZEKE) photoelectron spectroscopy is studied using the known wave‐packet dynamics of I2 (B state). The 340 fs wave‐packet period, wave‐packet dephasing and rephasing are observed in the ZEKE signal. The effect of various laser and ZEKE parameters on the wave‐packet dynamics is discussed.

High resolution photoelectron spectra of the NO dimer

High resolution zero kinetic energy (ZEKE) photoelectron spectra of the NO dimer are measured. They provide information about the ionization energy of the neutral, as well as about the binding energy, vibrations and structure of the ionized dimer indicating considerable structural reorganization of the dimer upon ionization.

Experimental Assessment of the Strengths of B–B Triple Bonds

Diborynes, molecules containing homoatomic boron-boron triple bonds, have been investigated by Raman spectroscopy in order to determine the stretching frequencies of their central B≡B units as an experimental measure of homoatomic bond strengths. The observed frequencies between 1600 and 1750 cm(-1) were assigned on the basis of DFT modeling and the characteristic pattern produced by the isotopic distribution of boron. This frequency completes the series of known stretches of homoatomic triple bonds, fitting into the trend established by the long-known stretching frequencies of C≡C and N≡N triple bonds in alkynes and dinitrogen, respectively. A quantitative analysis was carried out using the concept of relaxed force constants. The results support the classification of the diboryne as a true triple bond and speak to the similarities of molecules constructed from first-row elements of the p block. Also reported are the relaxed force constants of a recently reported diborabutatriene, which again fit into the trend established by the vibrational spectroscopy of organic cumulenes. As part of these studies, a new diboryne with decreased steric bulk was synthesized, and a computational study of the rotation of the stabilizing ligands indicated alkyne-like electronic isolation of the central B2 unit.

Photodissociation dynamics of the propargyl radical

The photochemistry and photodissociation dynamics of the propargyl radical, C3H3, upon UV excitation is investigated by time- and frequency-resolved detection of hydrogen atoms. From a statistical analysis of the data, we conclude that formation of cyclopropenylidene, c-C3H2, is the dominant reaction channel. Around 22% of the excess energy is released into the translational degrees of freedom. By varying the excitation energy between 265 and 240 nm, microcanonical rates for the loss of a hydrogen atom can be obtained as a function of excess energy. The experimental rates, on the order of several 106 s−1, are in good agreement with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations, provided a scaling factor is used for the vibrational frequencies, to account for the effects of anharmonicity. The interpretation is confirmed in experiments using monodeuterated propargyl radicals, H2CCCD, indicating a mechanism that proceeds via an initial [1,2] H-shift, followed by cyclization.

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.

Photodissociation dynamics of the allyl radical

The photochemistry and photodissociation dynamics of the allyl radical upon ultraviolet (UV) excitation is investigated in a molecular beam by using time- and frequency-resolved photoionization of hydrogen atoms with Lyman-α-radiation. The UV states of allyl decay by internal conversion to the ground state, forming vibrationally hot radicals that lose hydrogen atoms on a nanosecond time scale. Two channels are identified, formation of allene directly from allyl, and isomerization from allyl to 2-propenyl, with a subsequent hydrogen loss, resulting in both allene and propyne formation. The branching ratio is between 2:1 and 3:1, with direct formation of allene being the dominant reaction channel. This channel is associated with site-selective loss of hydrogen from the central carbon atom, as observed in experiments on isotopically labeled radicals. Ab initio calculations of the reaction pathways and Rice–Ramsperger–Kassel–Marcus (RRKM) calculations of the rates are in agreement with the mechanism and branching ratios. From the measured Doppler profiles a translational energy release of 14±1 kcal/mol is calculated. The calculated value of 66 kcal/mol for the barrier to the 1,2 hydrogen shift from allyl radical to 2-propenyl is confirmed by the experimental data.

State-to-state photoionisation dynamics probed by zero kinetic energy (ZEKE) photoelectron spectroscopy

High-resolution ZEKE (zero kinetic energy photoelectron) spectroscopy is a tool to study molecular photionisation processes with rotational resolution. Rotationally resolved ZEKE spectra for several molecules (NO, H2S and benzene) are discussed in detail and a qualitative model for the investigation of the state-selective ionisation dynamics is employed. This compound-state model provides a simple approach for the qualitative understanding of rotationally resolved ionisation processes and the underlying symmetry selection rules. The description is extended to the interpretation of the ZEKE spectra of benzene and C2v asymmetric rotor molecules (H2O, H2S). From these considerations the rotational selection rules that govern the ZEKE spectra of these and similar molecules can be predicted. A comparison with recent experimental results on H2S and new measurements on benzene demonstrates the importance of electronic symmetry selection rules in ZEKE spectroscopy.

The zero kinetic energy photoelectron spectrum of the propargyl radical, C3H3

We report the zero kinetic energy photoelectron spectrum of the propargyl radical, C3H3. From the spectrum an ionization energy of 69 953±10 cm−1 (8.673 eV) is deduced. Vibrational frequencies are obtained for the totally symmetric normal modes of the propargyl cation, as well as some combination and overtone bands. Both the frequencies and the relative intensities agree well with the predictions from recent ab initio calculations.

Dynamics of H-atom loss in adenine

Time- and frequency resolved detection of H-atoms was used to investigate the excited-state photophysics and photochemistry of isolated adenine and 9-methyladenine at two excitation wavelengths, 266 nm and 239.5 nm. By comparison of the two molecules it is shown that dissociation of the N9–H bond is an important deactivation pathway in adenine. The appearance time of the H-atoms was shorter than a few nanoseconds, indicating a rapid non-statistical dissociation. From the Doppler profiles a dissociation energy of 393 ± 20 kJ mol−1 was determined for adenine. Typically 40–50% of the excess energy is released as translation. The Doppler profile of the H-atom photofragments from adenine excited at 239.5 nm shows an anisotropic angular distribution and suggests that dissociation occurs in less than a rotational period. An anisotropy parameter β ≈ −0.9 was derived, indicating an almost pure perpendicular transition. Our data are consistent with a H-atom loss in adenine that can be adequately described by a coupling with πσ* excited states as recently suggested in the literature. The Doppler profiles of 9-methyladenine on the other hand are more isotropic. From the width of the profile it is assumed that not only one of the N–H bonds in the amino group is cleaved, but some H-loss also occurs on the methyl group.

Photoionization of three isomers of the C9H7 radical.

Three resonance-stabilized radicals, 1-indenyl (Ind), 1-phenylpropargyl (1PPR), and 3-phenylpropargyl (3PPR), all isomers of the composition C(9)H(7), were generated by jet flash pyrolysis. Their photoionization was examined by VUV synchrotron radiation. The mass spectra show a clean and efficient radical generation when the pyrolysis is turned on. To study the photoionization, photoion yield measurements and threshold photoionization spectroscopy techniques were applied. We determined adiabatic ionization energies (IE(ad)) of 7.53 eV for Ind, 7.20 eV for 3PPR, and 7.4 eV for 1PPR. Ab initio calculations show no major change in geometry upon ionization, in agreement with ionization from a nonbonding molecular orbital. The IEs were also computed and are in agreement with the measured ones. The difference in the IE might allow a distinction of the three isomers in flames. In the indenyl spectrum, an excited a(+) (3)B(2) state of the cation was identified at 8.10 eV, which shows a low-energy vibrational progression of 61 meV. Furthermore, we have examined the dissociative photoionization of the precursors. The indenyl precursor, 1-indenyl bromide, undergoes dissociative photoionization to Ind(+). An appearance energy (AE(0K)) of 10.2 eV was obtained from fitting the experimental breakdown diagram. A binding energy of 1.8 eV can thus be determined for the C-Br bond in 1-indenyl bromide. The phenylpropargyl precursors 1PPBr (1-phenylpropargyl bromide/3-phenyl-3-bromopropyne) and 3PPBr (3-phenylpropargyl bromide/1-phenyl-3-bromopropyne) also lose a bromine atom upon dissociative photoionization. Approximate appearance energies of 9.8 eV for 3PPBr and 9.3 eV for 1PPBr have been determined.