Far-infrared Laser Magnetic Resonance Research Articles

Collision-induced intersystem crossing of CH2 fromã 1A1 toX̃ 3B1: A case study of the mixed-state model

The rate coefficients for collision-induced intersystem crossing (CIISC) of methylene from the ã 1A1 first excited to the X̃ 3B1 ground electronic state, CH2 (ã 1A1)+M→CH2 (X̃ 3B1)+M, were investigated within the framework of the mixed-state mechanism [see, e.g., K. F. Freed, in Potential Energy Surfaces, edited by K. P. Law (Wiley, New York, 1980)]. Accordingly, the overall electronic relaxation was assumed to proceed via a sequence of rotational transitions within the ã manifold and allowed transitions from the ã to the X̃ manifold originating via ‘‘gate’’ states of ã which are states that contain some triplet character due to spin–orbit coupling with nearby X̃ rovibrational states. The perturbed ã and perturbing X̃ levels and relevant interaction matrix elements were identified from the available spectroscopic data. Rate coefficients for rotational relaxation processes were obtained from collision broadening measurements of CH2 (X̃) far-infrared laser magnetic resonance (FIR-LMR) transitions. Taking these data, thermal CIISC rate constants corresponding to experimental results for the overall depletion of rotationally thermalized CH2 (ã) and build up of (X̃) by M were evaluated for twelve interesting collision partners (M=He, Ne, Ar, Kr, Xe, N2, SF6, H2, D2, CH4, C2H6, H2O), taking into account every single CH2 (ã) rotation vibration state with energies up to Evr≤900 cm−1. The results were found to be in good agreement with reported room-temperature experimental data. Temperature dependencies, which were predicted for M=Ar, N2, and CH4, also agree with measured values.

Line-broadening measurements of CH 2 ( [formula omitted]) rotational transitions by far-infrared laser magnetic resonance

Collisional line-broadening (pressure-broadening) measurements have been performed using the method of Laser Magnetic Resonance (LMR) for selected rotational transitions of CH 2 ( X ̃ 3B 1) in the far-infrared with 11 interesting bath gases ( M = He, Ne, Ar, Kr, N 2, SF 6, H 2, D 2, CH 4, C 2H 6, and H 2O) at room temperature. In addition, the temperature dependence has been studied for M = Ar, N 2, and CH 4. Neglecting contributions from elastic rotational dephasing or reorientation collisions, the results correspond to relaxation cross sections between σ = 16 A ̊ 2 for M = He to σ = 195 Å 2 for H 2O.

The far-infrared laser magnetic resonance spectrum of the SH radical

We have extended the observations in the far-infrared laser magnetic resonance spectrum of the SH radical in its ground 2Π state to cover the first three rotational transitions in the 2 Π 3 2 component. Signals were seen from all naturally occurring isotopes of sulfur except the least abundant, 36S (0.02%). The measurements have been analyzed, together with selected data from previous work, to determine an improved set of molecular parameters for 32SH in its ground state. The implications of these parameters are discussed. The magnetic and electric nuclear hyperfine parameters for 33SH are significantly improved over previous work. The frequencies of rotational transitions in 32SH in the absence of a magnetic field are calculated from the parameter set to aid in the detection of this radical in astrophysical sources.

Kinetics of the Reaction of OH Radicals with CH2CO

AbstractThe kinetics of the homogeneous gas phase reaction OH + CH2CO → products (1) has been investigated at room temperature and pressures between 1.4 mbar ≤ p ≤ 2.5 mbar using the discharge flow method. Reactors with Teflon wall coatings and with different surface‐to‐volume ratios were employed to exclude effects of heterogeneous reactions. OH radicals were generated via the reaction F + H2O. Their concentration‐versus‐time decay profiles were followed under pseudo‐first‐order conditions, [CH2CO] → [OH]0 with a Far Infrared Laser Magnetic Resonance (FIR‐LMR) spectrometer. The rate constant of the title reaction was found to be k1, (296 K) = (7.2 ± 2.0)·1012 cm3/mol·s.

Detection of the HCS radical by far-infrared laser magnetic resonance

New far-infrared laser magnetic resonance (LMR) spectra from a free radical containing sulfur have been detected in reactions of fluorine, chlorine and hydrogen atoms with several thio-molecules. The same radical is also produced in the chain reaction between molecular fluorine and (CH3)2S2, (CH3)2S and mercaptan. The most extensive and intense LMR spectra were observed on the 108 µm D2O laser line. Based on chemical tests, isotopic substitution and qualitative spectroscopic considerations these spectra are assigned to the HCS radical. This is the first experimental observation of this radical in either the gas or condensed phases.

Far-infrared laser magnetic resonance spectrum of SO in the X3Σ − state

Far-infrared laser magnetic resonance spectrum of SO in the X3Σ − state

Molecular parameters of chromium hydride in its X6Σ + state determined by far-infrared laser magnetic resonance spectroscopy

The far-infrared laser magnetic resonance spectrum of the CrH radical in the v = 0 level of its X 6Σ + state has been studied in detail. Signals associated with the five lowest rotational transitions have been detected. Nearly 500 resonances for 52CrH have been assigned and used to determine molecular parameters which describe the rotational energy, the fine and proton hyperfine splittings, and the Zeeman effect. These parameters are well determined and their implications for the structure of CrH are discussed. A fourth-order correction to the spin-spin coupling has been identified for the first time. Precise zero-field rotational transition frequencies have been calculated which may allow the detection of the CrH radical in the interstellar medium.

Avoided crossings in the far-infrared laser magnetic resonance spectrum of HCO

Several transitions in the far-infrared spectrum of the HCO radical have been detected by the technique of laser magnetic resonance (LMR). The spectra have been assigned to both a- and b-dipole transitions and have been fitted together with previous microwave, millimeter-wave, and infrared measurements to determine a full set of parameters for HCO in its ground state. In particular, signals arising from the avoided crossing of the levels 8 08 and 7 16 in a magnetic field have been analyzed and used to determine values for the off-diagonal components of the spinrotation, proton dipolar hyperfine, and anisotropic g-tensors which are responsible for the mixing of the states. The reduced spin-rotation parameter ( ϵ ̃ ab + ϵ ̃ ba ) has been separated into its component parts by the imposition of the constraint ϵ ab ϵ ba = A B .

The rotational spectrum of the CH radical in its a 4Σ− state, studied by far-infrared laser magnetic resonance

The CH radical has been detected in its a 4Σ− state by the technique of laser magnetic resonance at far-infrared wavelengths. Spectra relating to different spin components of the first three rotational transitions have been recorded. The molecule was generated either by the reaction of F atoms with CH4, with a trace of added oxygen or by the reaction of O atoms with C2H2. The observed resonances have been analyzed and fitted to determine the parameters of an effective Hamiltonian for a molecule in a 4Σ state. The principal quantities determined are the rotational constant B0=451 138.434(94) MHz and the spin–spin parameter λ0=2785.83(18) MHz. Proton hyperfine parameters have also been determined.

Collision‐Induced Intersystem Crossing in CH2 (ã 1A1): A Quantitative Analysis Using the Mixed‐State Model

AbstractThe mechanism of collision‐induced intersystem crossing (ISC) in CH2 from the ã 1A1 first excited singlet to the X̃ 3B1, triplet ground electronic state, CH2 (ã) + M → CH2 (X̃) + M, is investigated. Collisional ISC rate constants are calculated for four selected collider gases, M = He, Ar, N2, and CH4, using the “mixed‐state” model of Freed and Gelbart. The most important triplet‐perturbed rotational levels in the ã‐state are identified from the spectroscopic data reported in the literature. The rates of rotational relaxation processes, which according to the model effectively govern the collision‐induced ISC, are determined by studying the collision broadening of CH2 (X̃) Far Infrared Laser Magnetic Resonance (FIR‐LMR) transitions. The collisional ISC rate constants for CH2 in the ortho nuclear spin modification are calculated to be kISC = 1.4 · 1012, 4.0 · 1012, 7.8 · 1012, and 6.6 · 1012 cm3/mol · s for M = He, Ar, N2, and CH4, respectively. The results are in good agreement with published experimental rate constants.

The far-infrared laser magnetic resonance spectrum of the CD radical and determination of ground state parameters

The far-infrared laser magnetic resonance spectrum of the CD radical in the v = 0 level of the X 2Π state has been studied in detail. Twelve transitions which are accessible with currently available laser lines have been recorded. The measurements have been analyzed and subjected to a single least-squares fit using an effective Hamiltonian. The data provide primary information on the rotational and fine-structure intervals between the lowest rotational intervals. They also yield values for the Λ-type doubling and deuteron hyperfine splittings in the same levels. Combination of the measurements with the corresponding data for CH allows the two parameters, γ and A D, to be determined separately.

Laser magnetic resonance rotational spectroscopy of the hydrogen halide molecular ions: H 35Cl + and H 37Cl +

Pure rotational spectra of the X 2Π vibronic ground states of H 35Cl + and H 37Cl + molecular ions have been measured by far-infrared laser magnetic resonance (LMR). The spectrometer, which contains an intracavity transverse dc discharge, is described in some detail. The theoretical formalism used to analyze the LMR spectra of HF +, HCl +, and HBr + is presented. The constants B, q, h, b, eQq 0, eQq 2, g 1, and g R were determined from a least-squares regression analysis of the J = 7 2 ← 5 2 and J = 9 2 ← 7 2 transitions in H 35Cl + and H 37Cl +. Values obtained for the molecular parameters support a model of HCl + with the electron density strongly localized on chlorine and with hydrogen exhibiting a substantial positive charge.

Kinetics of the reactions of methylene (X3B1)-radicals with nitric oxide and nitrogen dioxide

The kinetics of the reactions of CH{sub 2} radicals in their triplet electronic ground state ({tilde X}{sup 3}B{sub 1}) with NO and NO{sub 2} (CH{sub 2}({tilde X}{sup 3}B{sub 1}) + NO {yields} products (1); CH{sub 2}({tilde X}{sup 3}B{sub 1}) + NO{sub 2} {yields} products (2)) have been studied at room temperature in isothermal discharge flow systems. CH{sub 2} radicals were generated either by exciplex laser photolysis of CH{sub 2}CO or via the reaction O + CH{sub 2}CO {yields} CH{sub 2} + CO{sub 2}. The CH{sub 2} concentration was monitored directly with a far-infrared laser magnetic resonance spectrometer. From the decay of (CH{sub 2}) under pseudo-first-order conditions the rate constants of reactions 1 and 2 were obtained to be k{sub 1}(296K) = (2.2 {plus minus} 0.5) {times} 10{sup 13} cm{sup 3}/(mol s) and k{sub 2}(296K) = (5.9 {plus minus} 1.4) {times} 10{sup 13} cm{sup 3}/(mol s).

Erratum: Precise measurement of the J=2←1 fine structure interval in N(II) by far-infrared laser magnetic resonance [J. Chem. Phys. 8 4, 6101 (1986)

Erratum: Precise measurement of the <i>J</i>=2←1 fine structure interval in N(II) by far-infrared laser magnetic resonance [J. Chem. Phys. <b>8</b> <b>4</b>, 6101 (1986)

The far-infrared laser magnetic resonance spectrum of vibrationally excited C 2H

Some previously observed but unassigned lines in the laser magnetic resonance spectrum of the F + CH 4 flame at 490.4 μm have been assigned to the N = 7 ← 6, + ← − transition of the C 2H radical in its (010) vibrational level. The spectrum is an interesting example of a magnetic resonance spectrum of a radical displaying Hund's case (b) coupling and arises because the laser frequency is very close to the zero-field molecular frequency. The measured resonances are combined with recently available millimeter-wave observations of C 2H in the same vibrational level to give an improved set of molecular parameters. Some of these parameters provide quantitative information on the extent of the mixing of the X ̃ 2 Σ + state with the low-lying A ̃ 2 Π state of C 2H.

The microwave spectrum of the PH 2 radical

Five more rotational transitions of the PH 2 radical in the ground state were observed in the submillimeter-wave region. Sixty-one fine and hyperfine components were measured for the five transitions. An analysis of the observed spectra, combined with the microwave spectra reported in the previous paper, far-infrared and mid-infrared laser magnetic resonance spectra, and laser-induced fluorescence spectra obtained by other workers, improved the accuracy of most of the molecular constants given in the previous paper.

The far-infrared Laser Magnetic Resonance spectrum of the 17OH radical: Determination of nuclear hyperfine parameters

The far-infrared (FIR) Laser Magnetic Resonance (LMR) spectrum of the 17OH radical in the v = 0 level of the X 2Π state has been studied in detail. The measurements have been analyzed and, in combination with some earlier EPR measurements, subjected to a single least-squares fit using an effective Hamiltonian. A full set of 17O nuclear hyperfine parameters has been determined. Some implications of these parameter values for the electronic wavefunction of OH are considered.

A direct study of the reactions of methylene(~X 3B1) radicals with hydrogen and deuterium atoms

The following reactions of CH/sub 2/ radicals in their triplet electronic ground state X(tilde) /sup 3/B/sub 1/ with hydrogen and deuterium atoms were investigated at room temperature and pressures of a few mbar in an isothermal discharge flow reactor: CH/sub 2/(X(tilde) /sup 3/B/sub 1/) + H(/sup 2/S) ..-->.. CH(/sup 2/II) + H/sup 2/(/sup 1/..sigma..) (1a) ..-->.. CH/sub 3/(/sup 2/A/sub 2/'') (1b); CH/sub 2/(X(tilde) /sup 3/B/sub 1/) + D(/sup 2/S) ..-->.. CH(/sup 2/II) + HD(/sup 1/..sigma..) (2a), ..-->.. CD(/sup 2/II) + H/sub 2/(/sup 1/..sigma..) (2b), ..-->.. CHD(/sup 3/B/sub 1/) + H(/sup 2/S) (2c), ..-->.. CH/sub 2/D(/sup 2/A/sub 2/'') (2d). CH/sub 2/ and CH radicals were detected with a far-infrared laser magnetic resonance (LMR) spectrometer while H and D atoms were monitored via their Lyman-..cap alpha.. vacuum ultraviolet resonance absorption. Direct measurements of the pseudo-first-order decay of (CH/sub 2/) in the presence of an excess of H or D yielded the rate constants k/sub 1/(298 K) = (1.1 +/- 0.3) x 10/sup 14/ cm/sup 3//(mol s) and k/sub 2/(298 K) = (1.1 +/- 0.3) x 10/sup 14/ cm/sup 3//(mol s). Under the conditions used the recombination channels 1b and 2d are negligible. Thus, the measured values correspond to k/sub 1/ = k/submore » 1a/ and k/sub 2/ = K/sub 2a/ = k/sub 2b/ = k/sub 2c/. When the value for k/sub 1a/ is combined with literature data for the reverse reaction CH + H/sub 2/ ..-->.. CH/sub 2/ + H (-1a), the heat of formation of CH/sub 2/ radicals was found to be ..delta..H/sub f//sup 0/ /sub 298/(CH/sub 2/(X(tilde)/sup 3/B/sub 1/)) = 389 +/- 4 kJ/mol.« less

The far-infrared laser magnetic resonance spectrum of the CF radical and determination of ground state parameters

Observations in the far-infrared laser magnetic resonance spectrum of the CF radical in its ground 2Π state have been extended to include fine structure transitions between the two spin components. The data are fitted together with all previous measurements relating to the v = 0 level to obtain a complete set of molecular parameters, including the spin-orbit splitting which has been determined at 77.196916(14) cm −1. The implications for the electronic structure of various parameters are also discussed.

Accurate determination of the fine-structure intervals in the 3P ground states of C-13 and C-12 by far-infrared laser magnetic resonance

Accurate values are presented for the fine-structure intervals in the 3P ground state of neutral atomic C-12 and C-13 as obtained from laser magnetic resonance spectroscopy. The rigorous analysis of C-13 hyperfine structure, the measurement of resonant fields for C-12 transitions at several additional far-infrared laser frequencies, and the increased precision of the C-12 measurements, permit significant improvement in the evaluation of these energies relative to earlier work. These results will expedite the direct and precise measurement of these transitions in interstellar sources and should assist in the determination of the interstellar C-12/C-13 abundance ratio.