Wen Bih Tzeng

Kinetic energy release measurements of ammonia cluster ions during metastable decomposition and determination of cluster ion binding energies

The dissociation dynamics of protonated ammonia clusters is investigated following their production from neutrals using multiphoton ionization. A very useful method of measuring the kinetic energy release arising from evaporative unimolecular dissociation is established; it employs a time‐of‐flight/reflectron to separate daughters and parents and enables the energy release to be determined from the peak shapes. The method is found to lead to values of high precision. Using a modified statistical theory analysis suggested by Engelking, it is shown that binding energies for large cluster ions can be readily determined. The results compare very well with those derived by other methods, ones which generally have much more severe size limitations. In the case of (NH3)n H+, the observed drop in binding energy from n=5 to 6 is consistent with the particularly stable protonated pentamer structure. Heretofore, unavailable results are available from n=7 to 17, with hints of slightly more stable cluster ions at n=12...

Mass analyzed threshold ionization of deuterium substituted isotopomers of aniline and p-fluoroaniline: Isotope effect and site-specific electronic transition

Two-color resonant two-photon mass analyzed threshold ionization (MATI) spectroscopy was used to record the threshold ion spectra of deuterium-substituted isotopomers of aniline and p-fluoroaniline. The respective adiabatic ionization energies of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, C6D5ND2, p-FC6H4NH2, p-FC6H4NHD, and p-FC6H4ND2 were determined to be 62 271, 62 253, 62 233, 62 258, 62 237, 62 214, 62 543, 62 520, and 62 507 cm−1 with an uncertainty of about 5 cm−1. Analyses on the shifts in the electronic transition and ionization energies prove that the S1←S0 transition mainly occurs around the aromatic ring whereas the transition from the neutral S1 to the cationic state corresponds to the removal of one of the lone-pair electrons of nitrogen. The present results provide the first experimental evidence for the site-specific electronic transition in aniline as well as the isotope effects on the vibrations of the aniline cation. These findings are well supported by ab initio and density functiona...

A 193 nm laser photofragmentation time‐of‐flight mass spectrometric study of CS2 and CS2 clusters

A crossed laser and molecular beam photofragmentation apparatus is described. The apparatus is equipped with a rotatable molecular beam source and a translationally movable ultrahigh vacuum mass spectrometer for time‐of‐flight (TOF) measurements. Using this apparatus we have measured the TOF spectra of S and CS resulting from the photofragmentation processes, CS2+hν(193 nm)→CS(X,v)+S(1D or 3P). The translational energy distributions of photofragments derived from the S and CS TOF spectra are in good agreement. This observation, together with the finding that the TOF spectra of S and CS are independent of laser power in the 25–150 mJ range, shows that the further absorption of a laser photon by CS to form C(3P)+S(3P) within the laser pulse is insignificant. The TOF spectra of S obtained at electron ionization energies of 20 and 50 eV are indiscernible, indicating that the contribution to the TOF spectrum of S from dissociative ionization of CS is negligible at electron impact energies ≤50 eV. The thermoche...

A study of the structures and vibrations of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, and C6D5ND2 in the S1 state by ab initio calculations

Abstract The structures and vibrations of C6H5NH2, C6H5NHD, C6H5ND2, C6D5NH2, C6D5NHD, and C6D5ND2 have been studied using ab initio molecular orbital (MO) calculations. Calculations at the CIS/6-31+G∗ level successfully predict that aniline is planar in the S1 state, and contracts along the long in-plane axis with a quinoid-like resonance structure. A comparison of the S0 and S1 state vibrational modes is presented and discussed in terms of the changes in reduced masses, force constants, and mode mixing for aniline and its isotopic species. The computed frequencies are found to be in very good agreement with the available experimental values.

Dissociation dynamics: Measurements of decay fractions of metastable ammonia cluster ions

The decay fractions of metastable ammonia cluster ions which undergo unimolecular (evaporative) dissociation in a time window of 1–40 μs were measured by using a reflection time‐of‐flight mass spectrometer. Corrections concerning instrumental artifacts and ion trajectory of parents and daughters are made to imporve the precision of the measurements. The data are used to derive the Gspann parameter and heat capacity of clusters as described in evaporative ensemble model of metastable dissociation. Using the dissociation fractions measured in the present studies, in conjunction with kinetic energy release values previously measured in our laboratory, we apply Klots’ evaporative ensemble model to obtain binding energies of ammonia cluster ions (NH3)nH+, 4≤n≤17. The deduced binding energy values are found to be in very good agreement with both thermochemical data and Engelking’s modified statistical theory.

A Gaussian‐2 ab initio study of CH2SH, CH2S−, CH3S−, CH2SH−, CH3SH−, CH3+, and CH3SH+

Using the Gaussian‐2 (G2) theoretical procedure, we have examined the molecular structures and total energies for CH2SH, CH2S−, CH3S−, CH2SH−, CH3SH−, CH3+, and CH3SH+. Contrary to the relative stabilities of CH3S+(C3v;3A2) and CH2SH+(Cs;1A’), the methylthio radical CH3S(Cs;2A’) and the methylthio anion CH3S−(C3v;1A1) are predicted to be more stable than the mercaptomethyl radical CH2SH(C1;2A) and the mercaptomethyl anion CH2SH−(Cs;1A’) by 9.2 and 38.0 kcal/mol, respectively. The CH2SH−(Cs;1A’) anion may exist in the cis configuration or the less stable trans structure. Combined with the results of previous G2 calculations, this calculation yields predictions for the adiabatic ionization energies (IE) of CH3 (9.79 eV), CH2SH (7.41 eV), and CH3SH (9.55 eV), which are in accord with the experimental IEs of 9.84 eV for CH3, 7.536±0.003 eV for CH2SH, and 9.440 eV for CH3SH. The G2 values for the adiabatic electron affinities (EA) of CH2S, CH2SH to trans‐CH2SH−(Cs;1A’), CH2SH to cis‐CH2SH−(Cs;1A’), and CH3S ar...

Spectroscopy of phenylacetylene bound to clusters of ammonia and the surface cluster analogy

The results of a systematic study of the spectra shifts of the S1–S0 transition in phenylacetylene (PA) [1B2(1Lb)←1A1(1A)] due to solvation by NH3 are reported based on two‐photon REMPI. A unique aspect of the present work is comparison of the red shifts for clusters produced by coexpansion techiques with those where PA is attached to performed ammonia clusters. In both experiments the 1–1 complex is red shifted by 80 cm−1. In the case of the coexpansion experiments the red shift is about 50 cm−1 for PA(NH3)n, n=2–7. By contrast, the clusters produced by attachment display a much different and gradually increasing red shift which saturates at 150 cm−1 for n=13 and beyond. Structural differences for clusters produced by the two different techniques are believed to be responsible.

Structures and vibrations of o-methylaniline in the S0 and S1 states studied by ab initio calculations and resonant two-photon ionization spectroscopy

Abstract Ab initio calculations show that the optimized structure of o-methylaniline (OMA) in the ground state is non-planar with the amino part having sp3 hybridization-like character due to the existence of lone-paired electrons on the nitrogen atom. In the S1 state, the C−N bond of OMA exhibits a partial double bond character, indicating an enhanced interaction between the ring and the NH2 group. In addition, the NH2 and the CH3 substituents are repelled from each other such that the hindrance of the methyl rotation by the amino group is greatly reduced. Our experimental results from jet-cooled resonant two-photon ionization (R2PI) spectroscopy show that most active transitions are related to the in-plane ring vibrations and the methyl rotations. Spectral features corresponding to the in-plane substituent-sensitive stretching and bending vibrations of OMA in the S1 state are observed for the first time. All the observed spectral bands have been successfully assigned on the basis of our computed results and comparison with those of other relevant substituted benzenes.

Mass analyzed threshold ionization of the 35Cl and 37Cl isotopomers of p-chloroaniline

One-color, two-color resonant two-photon ionization (R2PI), and mass analyzed threshold ionization (MATI) spectroscopic methods have been used to study the electronic transition and the threshold ionization of the 35Cl and 37Cl isotopomers of p-chloroaniline. The band origins of the S1←S0 electronic transitions of both species are found to be 32 573±1 cm−1. The ionization energies (IEs) of both isotopomers of p-chloroaniline are determined to be 62 409±7 cm−1 by the two-color R2PI spectroscopy and 62 410±4 cm−1 by the MATI spectroscopy. Analyses on the spectral features show that most of the active modes are related to the in-plane ring vibrations in the S1 state and cationic ground state. Isotope effect on the ring deformation vibrations 1, 6a, and 12 gives rise to a frequency shift of 1–3 cm−1 in the S1 state and 3–9 cm−1 in the ion state. The experimental results are well supported by the computed ones on the basis of ab initio and density functional theory calculations.

Species-Selected Mass-Analyzed Threshold Ionization Spectra of m -Fluoroaniline Cation

Two-color resonant, two-photon mass-analyzed threshold ionization (MATI) spectroscopy was used to probe the ionic properties of m-fluoroaniline (MFA). The species selection from a mixture was achieved by tuning the frequency of the excitation laser to a specific intermediate level of MFA for the successive excitation, followed by pulsed field ionization. The MATI spectra were recorded by ionizing via the 00 vibrationless and the 41, 151, and 9b1 vibrational levels in the S1 state. The adiabatic ionization energy of MFA was determined to be 64 159 ± 5 cm−1 (7.9542 ± 0.0006 eV). Analysis of the spectral features shows that the active modes of the ion are closely related to the specified vibrations of the neutral. These findings indicate that the geometry of the ion is quite similar to that of the neutral in the S1 state.