Jeffrey W. Hudgens

Two photon resonance enhanced multiphoton ionization spectroscopy and state assignments of the methyl radical

A mass resolved multiphoton ionization spectroscopy study of CH3 and CD3 has revealed two new electronic Rydberg series in these radicals. Simultaneous absorption of two photons prepared the excited states. Absorption of a third photon from the excited state generated the parent ions of methyl radical (m/z 15; CH3 and m/z 18; CD3) which were detected in a quadrupole mass spectrometer. The np 2A″2 Rydberg series was detected and identified by a rotational assignment of the 3pu20092A″2 state of CD3 and by a polarization study of the np states. The quantum defect of this series was 0.6. The rotational constants determined for the ground and 3pu20092A″2 excited states were B″0=4.798 cm−1 and B′0=4.768 cm−1. A vibrational analysis shows that both the symmetric stretch and out‐of‐plane bending mode are active in the 3pu20092A″2 state. A second Rydberg series with a quantum defect of zero was detected and tentitively assigned to the nf 2E′ series.

ArF excimer laser multiphoton-ionization mass spectrometry of organic molecules

Abstract Mass spectra are obtained for 27 organic molecules following irradiation with focussed 193-nm (6.42-eV) photons from an ArF excimer laser. The molecules chosen represent several classes of organic compound. Comparisons between the multiphoton-ionization mass spectra and low-energy electron-impact mass spectra indicate that the dominant ionization pathway is non-resonant two-photon absorption. Data for benzaldehyde and CS 2 provide clear exceptions to this mechanism. These two molecules exhibit enhanced fragmentation owing to the presence of one-photon resonances.

Production, detection and reactions of the CH radical

Abstract We report the production of CH (X 2 H) radicals by the multiphoton dissociation of CHBr 3 at 193 nm, the probing of these radicals by laser-induced fluorescence on the (A 2 Δ — X 2 H) transition, and the measurement of their reaction rates with H 2 , CH 4 and N 2 .

Discrete and quasicontinuum level fluorescence from infrared multiphoton excited SF6

Infrared laser multiphoton excited SF6 shows infrared fluorescence spectra characteristic of the laser excitation process. These fluorescence experiments were conducted under dilute conditions so that the effects of collisions are essentially eliminated. After laser preparation fluorescence from molecules populating the quasicontinuum and discrete level ensembles are observed. These ensembles show distinctly different spectral characteristics which help demark energy zones which these types of levels occupy. Discrete levels which exist at lower vibrational energies show sharp fluorescence characteristic of energy localized in the laser excited mode. Quasicontinuum levels which exist at chemically interesting energies reveal that collisionless intramolecular relaxation has occured. Model calculations on thermally prepared and infrared laser prepared SF6 molecules allow the assignment of the average energy deposition into SF6 as a function of fluence, and of the population distribution of SF6 within the qua...

Observation of OH (υ = 0, 1) in the reactions of O(3P) with HCl (υ = 0, 1, 2)

Abstract The analysis of the observed OH (υ = 0, 1) concentration in the laser enhanced reactions of HCl (υ = 1, 2) with O(3P) atoms demonstrates that vibrational energy in excess of the thermal activation energy barrier continues to enhance the reaction rate. This reaction also exhibits a preferential conversion of reactant vibrational excitation into product vibrational excitation.

Detection of CH/sub 2/OH radicals by resonance-enhanced multiphoton ionization spectroscopy

The hydroxymethyl radical, CH/sub 2/OH, has been detected by the method of resonance-enhanced multiphoton ionization mass spectroscopy. Radicals were produced via hydrogen abstraction from CH/sub 3/OH by fluorine atoms in a discharge flow reactor. Tunable laser radiation between 427 and 490 nm ionized the CH/sub 2/OH radicals. The identity of the radical was determined from mass spectrometry of deuterium-substituted radicals. The isomerization reaction which transforms methoxy radicals into hydroxymethyl radicals was not observed.

Detection of gas-phase methyl radicals using multiphoton ionization

Abstract Detection of methyl radicals by the method of mass-selected multiphoton ionization is demonstrated by observing CH 3 Rydberg state resonances. The Rydberg states were prepared by simultaneous absorption of three photons. Absorption of a fourth photon produced the observed mass 15 ion signals.

Electronic spectra of the heteroisotopic CH2D and CHD2 radicals by resonance enhanced multiphoton ionization

The 3pu20092B1←←Xu20092B1 bands of CH2D and CHD2 radicals were observed between 305 and 335 nm by mass resolved, 2+1 resonance enhanced multiphoton ionization spectroscopy. Spectroscopic constants were found for the 3pu20092B1 Rydberg state of the CH2D radical (ν00=59u2009940 cm−1, ν1a1 CH2 stretch=2995 cm−1, ν2a1 CD stretch=2220 cm−1, ν4b1 out‐of‐plane large amplitude (OPLA)=1260 cm−1, ν5b2 CH2 asymmetric stretch=3055 cm−1, ν6b2 CD bend=1115 cm−1) and of the CHD2 radical (ν00=59u2009920 cm−1, ν1a1 CH stretch=3040 cm−1, ν2a1 CD2 stretch=2150 cm−1, ν4b1 OPLA=1165 cm−1, ν6b2 CH bend=1210 cm−1). Vibrational frequencies calculated by ab initio theory agree well with the experimental data.

New electronic states in CH3, observed using multiphoton ionization

We report the first observation of the npe′ (2E′) and npa″2 (2A″2) electronic states in gas phase methyl radicals using the technique of resonance enhanced multiphonon ionization (REMPI). These previously unobserved states were identified in our REMPI spectrum as two‐photon allowed resonance transitions leading to ionization. These electronic states arise from the excitation of the ground state electron in CH3 to np Rydberg orbitals. Transitions into these states from the ground 2A″2 electronic state are forbidden in one photon; however, they are allowed in a two‐photon absorption. (AIP)

Identification of Active Sites of Biomolecules. 1. Methyl-α-mannopyranoside and FeIII

Car-Parrinello molecular dynamics (CPMD) simulations, DFT chemical reactivity index calculations, and mass spectrometric measurements are combined in an integrated effort to elucidate the details of the coordination of a transition-metal ion to a carbohydrate. The impact of the interaction with the FeIII ion on the glycosidic linkage conformation of methyl-alpha-d-mannopyranoside is studied by classical molecular dynamics (MD) and CPMD simulations. This study shows that FeIII interacts with specific hydroxyl oxygen atoms of the carbohydrate, affecting the ground state carbohydrate conformation. These conformational details are discussed in terms of a set of supporting experiments involving electrospray ionization mass spectrometry, and CPMD simulations clearly indicate that the specific conformational preference is due to intramolecular hydrogen bonding. Classical MD simulations proved insensitive to these important chemical properties. Thus, we demonstrate the importance of chemical reactivity calculations and CPMD simulations in predicting the active sites of biological molecules toward metal cations.