Dmitry G. Melnik

The spectroscopic characterization of the methoxy radical. I. Rotationally resolved à A21–X̃ E2 electronic spectra of CH3O

New experimental data have been obtained for the methoxy radical by observing at high-resolution laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) transitions between the X (2)E and lowest excited A (2)A(1) state. The SEP transitions were from the A state (pumped from the X (2)E(3/2) spin-orbit component) to the X (2)E(1/2) spin-orbit component. These data for the first time directly connect with high precision the spin-orbit components of the X (2)E ground state of CH(3)O. Surprisingly these new SEP observations are inconsistent with predictions of the X state structure based on long-standing analyses primarily based on the microwave spectra of ground state CH(3)O. It is found that all the experimental data can be understood consistently when the previously accepted value of the spin-orbit coupling constant is adjusted and the reflection parity assignments in the X state are reversed. The latter action changes the sign of a number of reflection-parity-dependent parameters in the X state. The ramifications of the changes and the physical interpretation of the resulting parameters are discussed in some detail.

The absorption spectroscopy of the lowest pseudorotational states of tetrahydrofuran

The rotational structure of the pseudorotational (PR) band n=0→n=2 has been observed in jet-cooled tetrahydrofuran in the 170–360 GHz frequency range. The observed transitions were analyzed together with the previously obtained microwave data of Meyer and co-workers [R. Meyer, J. C. Lopes, J. L. Alonso, S. Melandri, P. G. Favero, and W. Caminati, J. Chem. Phys. 111, 7871 (1999)]. The experimentally observed transitions provide direct spectroscopic evidence of the symmetry ordering of the lowest four observed PR states. Based on the symmetry properties of the pseudorotational states involved in this study, an analytical model of the potential energy surface (PES) along the pseudorotational path has been proposed that provides a consistent explanation of all the observed transition frequencies, including those from the early IR work. In addition, an analysis of the variation of the rotational constants of the molecule in different PR states has been performed using the proposed model. This analytical PES an...

Submillimeter wave vibration–rotation spectroscopy of Ar⋅CO and Ar⋅ND3

A new absorption spectrometer operating in the submillimeter wave region has been designed. The apparatus combines the previously reported fast scan submillimeter wave spectroscopic technique (FASSST) and a pulsed supersonic jet sample. It is specially designed for the rapid searching for unknown molecular transitions of weakly absorbing and/or low abundance species. Vibration–rotation transitions of the Ar⋅CO and Ar⋅ND3 van der Waals molecules have been observed using this apparatus. Transitions in Ar⋅CO involve the j=1→j=2 hindered rotor transition in the complex. Transitions in Ar⋅ND3 have been assigned as the rotational structure of the fundamental of the lowest frequency bending mode involving the two moieties. Transitions have been observed emanating from the levels correlating to the two lowest energy inversion components of ND3 yielding a determination of the inversion splitting in the complex.

The spectroscopic characterization of the methoxy radical. III. Rotationally resolved Ã2A1–X̃2E electronic and X̃2E submillimeter wave spectra of partially deuterated CH2DO and CHD2O radicals

Rotationally resolved laser induced fluorescence and stimulated emission pumping Ã(2)A(1)-X̃(2)E spectra, along with pure rotational spectra in the 153-263 GHz region within the E(3/2) component of the ground state in asymmetrically deuterated methoxy radicals CH(2)DO and CHD(2)O have been observed. The combined data set allows for the direct measurement with high precision of the energy separation between the E(1/2) and E(3/2) components of the ground state and the energy separation between the parity stacks in the E(3/2) component of the ground state. The experimentally observed frequencies in both isotopologues are fit to an effective rotational Hamiltonian accounting for rotational and spin-rotational effects arising in a near-prolate asymmetric top molecule with dynamic Jahn-Teller distortion. Isotopic dependencies for the molecular parameters have been successfully implemented to aid the analysis of these very complex spectra. The analysis of the first and second order contributions to the effective values of molecular parameters has been extended to elucidate the physical significance of resulting molecular parameters. Comparisons of measured parameters, e.g., spin-orbit coupling, rotational and spin-rotation constants, are made among the 5 methoxy isotopologues for which data is now available. Comparisons of experimental results, including the derived geometric structure at both the C(3v) conical intersection and at the Jahn-Teller distorted minima, are made with quantum chemistry calculations.

Measurements of the Absolute Absorption Cross Sections of the Ã←X̃ Transition in Organic Peroxy Radicals by Dual-Wavelength Cavity Ring-Down Spectroscopy

We demonstrate an experimental method for the accurate measurement of the absorption cross section of transient species, such as organic peroxy radicals in which the concentration of the radicals is determined via the absorption of a stable coproduct that is produced stochiometrically. The requirements for the experimental apparatus, a dual-wavelength cavity ring-down spectrometer, and the chemical protocol for transient species generation are discussed. The capability of this approach is demonstrated by measuring the peak absorption cross section of the ethyl peroxy radical, C₂2H₅O₂, whose value for the Ã←X electronic transition at 7596 cm⁻¹ (λ = 1316.5 nm) is found to be σ(p)(EP) = 5.29(20) × 10⁻²¹ cm². These present results are compared to those obtained from other methods of measurement of σ(p)(EP). Possible random and systematic errors are discussed.

Kinetic measurements of the C2H5O2 radical using time-resolved cavity ring-down spectroscopy with a continuous source

We report on the design of a time-resolved, high duty-factor cavity ring-down apparatus utilizing a continuous laser and detail a technique for the accurate and precise measurement of effective reaction rate constants with it. This report complements an earlier paper concerning the measurement of the absolute absorption cross-sections, σP, of reactive intermediates. To demonstrate the performance of the new technique, we have measured the decay rate of ethyl peroxy radicals by monitoring the Ã←X̃ origin band of the G-conformer of these species. A measured value kobs∕σP = 1.827(45) × 10(7) cm/s was determined and it, along with the previously measured value of σP, was used to derive the value of kobs = 9.66(44)×10(-14) cm(3)/s, for the effective rate constant for ethyl peroxy self-reaction (all uncertainties are 1 σ). The present value of kobs is compared to those previously reported, and sources of systematic errors and their impact are discussed.

Electronic transition moment for the 0(0)(0) band of the à ← X̃ transition in the ethyl peroxy radical.

The electronic transition moment for the G-conformer of ethyl peroxy was determined from the experimentally measured value of the peak absorption cross-section and the simulation of its rovibronic spectrum using the results of the high resolution spectroscopy of this molecule. The resulting value is |μ(e)(G)| = 2.55(6) × 10(-2) Debye, which is compared to values from electronic structure calculations.

The Changing Shapes of Molecules

A new microwave spectrometer enables the geometries of molecules to be tracked as they interconvert between different shapes.

ROVIBRONIC ANALYSIS OF THE e′ BANDS IN THE Ã2 E′′ STATE OF NO3 RADICAL

The vibronic structure of the NO3 radical has been the subject of much recent research in our group.a We have also collected several high resolution spectra of transitions to the Ã2E′′ state. Parallel bands, with a′′ 1 symmetry, have been satisfactorily fit using an oblate symmetric top Hamiltonian with spin rotation. Some lines were seen to be perturbed and it is likely that this is the result of random perturbations from levels originating from the ground electronic state. The perpendicular bands, which have e′ symmetry, are not satisfactorily described using this Hamiltonian. In particular the rotational structure of the e′ levels has many more transitions than in the oblate top model predicts. For this reason we have developed two different rovibronic Hamiltonians for the analysis of the vibronically degenerate levels. Both include spin-orbit, coriolis, spin-rotation, and Jahn-Teller distortion terms. However, they are derived starting from two different limiting cases. In Case 1 the Hamiltonian is built by assuming first a D3h configuration and then perturbations are added. Case 2 starts at the statically distorted, low symmetry geometry and introduces interactions among the vibronic levels. In the case of Jahn-Teller coupling that is neither very weak nor very strong these models should both adequately describe the observed spectra. These models and preliminary analysis of several e′ bands are presented.

Wide-field four-channel fluorescence imager for biological applications

A wide-field four-channel fluorescence imager has been developed. The instrument uses four expanded laser beams to image a large section (6 mm x 9 mm). An object can be sequentially illuminated with any combination of 408-, 532-, 658-, and 784-nm lasers for arbitrary (down to 1 ms) exposure times for each laser. Just two notch filters block scattered light from all four lasers. The design approach described here offers great flexibility in treatment of objects, very good sensitivity, and a wide field of view at low cost. There appears to be no commercial instrument capable of simultaneous fluorescence imaging of a wide field of view with four-laser excitation. Some possible applications are following events such as flow and mixing in microchannel systems, the transmission of biological signals across a culture, and following simulations of biological membrane diffusion. It can also be used in DNA sequencing by synthesis to follow the progress of the photolytic removal of dye and terminator. Without utilizing its time resolution, it can be used to obtain four independent images of a single tissue section stained with four targeting agents, with each coupled to a different dye matching one of the lasers.