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The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH2 (ã 1A1) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH2 is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH2 population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times. [ABSTRACT FROM AUTHOR]

The spectrum of methylene in the 1.3–1.4 and 0.89–0.94 μm wavelength regions has been recorded in absorption using frequency-modulated cw diode and Ti:sapphire laser sources. The spectral lines have Doppler-limited resolution and have been assigned to bands in the b 1B1←ã 1A1 electronic spectrum of the radical. In three of the four bands studied, the lower state is the bend excited, υ2″=1, level of the ã state and two of the upper levels lie below the energy of the degenerate linear configuration of the b/ã pair. Together with previously measured data pertaining to υ2″=1, the data have been used to refine the precision of the experimentally determined rotational structure in this level. Although several K″=1 levels do show shifts of more than 0.1–0.2 cm-1, multiple strong perturbations due to near-resonant background X 3B1 rovibrational levels, such as are known to occur in the ã 1A1, υ2″=0 level have not been found in υ2″=1. Absorption lines due to the predominantly triplet X(040) 414 level, responsible for most of the perturbation of ã(010) 515, have been identified in the spectra. The data also fix the energies of the b(0,0,0)2, ã(0,7,0)1, b(0,2,0)3, and ã(0,10,0)2 upper vibronic levels, where the numbers in parentheses are the vibrational quantum numbers with superscript K, the projection of the total angular momentum on the a-inertial axis. [ABSTRACT FROM AUTHOR]

Rotationally resolved spectra of several bands lying to the red of the origin of the ùA"-...A' band system of chloromethylene (HCCl), were recorded by laser absorption spectroscopy in ambient temperature and jet-cooled samples. The radical was made by excimer laser photolysis of dibromochloromethane, diluted in inert gas, at 193 nm. The jet-cooled sample showed efficient rotational but less vibrational cooling. Analysis showed that the observed bands originate in the (υ1, υ2, υ3)=(010), (001), and (011) vibrational levels of the ground electronic state of the radical, while the upper-state levels involved were (000), (010), (001), and (011). Vibrational energies and rotational constants describing the rotational levels in the lower-state vibrational levels were determined by fitting to combination differences. The analysis also resulted in a reevaluation of the C-Cl stretching frequency in the excited state and we find E'001=13 206.57 or 926.17 cm-1 above the ÃA"(000) rotationless level for HC35Cl. Scaled ab initio potential energy surfaces for the à and ... states were used to compute the transition moment surface and thereby the relative intensities of different vibronic transitions, providing additional support for the assignments and permitting the prediction of the shorter wavelength spectrum. All the observed upper state levels showed some degree of perturbation in their rotational energy levels, particularly in Ka=1, presumably due to coupling with near-resonant vibrationally excited levels of the ground electronic state. Transitions originating in the low-lying ã³A" were also predicted to occur in the same wavelength region, but could not be identified in the spectra. [ABSTRACT FROM AUTHOR]

The rotationally resolved spectrum of the HCCl Ã1A″(0,0,0)←X1A′(0,0,0) transition between 12 230 and 12 410 cm-1 was obtained for the first time at Doppler-limited resolution using a frequency-modulation (FM) transient absorption technique. Five major subbands (Ka = 0←1, 1←0, 1←2, 2←1, and 2←3) of HC35Cl and one subband (Ka = 0←1) of HC37Cl were observed and assigned. In addition, the forbidden subband (Ka = 0←0) of HC35Cl was also observed. Thus far, we have assigned most (more than 70%) of the strong lines. The analysis of these subbands determines the geometry for the HCCl Ã(0,0,0) level: /_ HCCl = 134.7° and RC-Cl = 1.623 Å while RC-H is fixed at the corresponding value of CH2. The height of the barrier to linearity was found to be approximately 2170 cm-1 above the vibrationless level of the à state. The rotational structure of this spectrum shows irregular perturbations. Both random and J-dependent (anomalous K-type doubling) effects were observed. As in methylene, the perturbations most likely arise from Renner-Teller effect between the two singlet states in combination with spin–orbit coupling between the singlet and triplet states. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The infrared absorption spectrum of the ν3 fundamental band of the CD3 radical has been detected by diode laser absorption spectroscopy. The CD3 radical was produced by excimer laser photolysis of CD3I at 248 nm or (CD3)2CO at 193 nm. Molecular parameters of the v3=1 vibrational state were determined from a least-squares fit to 62 rotation–vibration transitions. In this fit, molecular parameters describing the ground state were constrained to those obtained from previous spectroscopic studies of the ν2 parallel IR band [J. M. Frye, T. J. Sears, and D. Leitner, J. Chem. Phys. 88, 5300 (1988)]. The molecular parameters determined in the present work are the band origin ν0=2381.088 60(84), B’=4.758 737(40), C’=2.373 297(34), (ζC)3=0.476 278(72), q3=0.003 76(59), D’N =0.000 187 9(5), DNK =-0.000 341 0(12), D’K =0.000 143 7(8), ηN =-0.000 005 5(36), η’K =0.000 060(35), and qN =0.000 063(17), all in cm-1 with one standard deviation in parentheses. The derived molecular parameters were compared with those for the CH3 radical v3=1 level determined previously [T. Amano, P. Bernath, C. Yamada, Y. Endo, and E. Hirota, J. Chem. Phys. 77, 5284 (1982)]. The molecular parameters of the v3=1 state of the CD3 and CH3 radicals follow the expected isotopic relationships. We have also found that the determined molecular parameters reasonably satisfy the approximate planarity relationships [J. K. G. Watson, J. Mol. Spectrsoc. 65, 123 (1977)] and the sign of the l-type doubling constant is consistent with a planar equilibrium structure. [ABSTRACT FROM AUTHOR]

CD3 radicals were generated by excimer laser photolysis (248 nm) of CD3 I, and the transient populations detected by diode laser absorption using a gated integrator signal averager. The v2 =1←0, 2←1, and 3←2 bands of the CD3 radical were observed, and band center frequencies were determined to be 457.8133(11), 507.9311(19), and 542.4510(29) cm-1, respectively. Observed transitions were fitted, by least squares analysis, to yield rotational and centrifugal distortion constants and spin–rotation parameters, and results are compared to theory. Measurements were also carried out of the time dependence of the vibration–rotation level populations as a function of vibrational excitation and spin component. [ABSTRACT FROM AUTHOR]

A Doppler-limited high resolution vibronic spectrum of the methylene (CH2) b 1B1←a 1A1 transition in the near-infrared wavelength region has been obtained using transient absorption techniques. The radical was produced by 308 nm excimer laser photolysis of ketene (CH2CO) in a flow system. The analysis of this spectrum confirms the reassignments of some previously observed bands as well as the presence of new vibronic bands predicted by a recent ab initio calculation [Green et al., J. Chem. Phys. 94, 118 (1991)]. We also measured the Doppler broadened profiles of low-J rotational lines of CH2 under collisionless conditions. From the line profile analysis, we find that the 308 nm photolysis of ketene produces fragments with isotropic and uncorrelated velocity and angular momentum distributions. The Doppler profiles also provide a view of the coincident product state distributions. The measured Doppler profiles are consistent with a rotational distribution of CO produced in coincidence with low energy CH2 states given by statistical phase space theory. The vibrationally excited coincident CO appears, however, to be produced about 2–3 times more efficiently than is predicted by various statistical theories. A frequency modulation (FM) technique has been adopted to improve the sensitivity of the transient absorption experiment. There is a significant improvement in the observed signal to noise ratio of the CH2 spectrum over the dual beam method previously used. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the observation and assignment of a number of vibration–rotation transitions within 1-←1+ band of H3O+. The data are analyzed together with previously published measurements to yield a consistent set of rotational parameters that accurately reflects the vibration–rotation spectra involving the four lowest levels associated with the ν2 vibration of the ion. The analysis enables a precise estimate of the inversion splitting and the associated inversion-rotation spectrum of H3O+ which occurs in the far infrared. [ABSTRACT FROM AUTHOR]

Abstract: The rotationally resolved spectrum of the (060)0–(000) band in the system of HCBr has been recorded in absorption using a frequency-modulated c.w. tunable dye laser. Both 79Br and 81Br isotopomer spectra were rotationally assigned and effective rotational constants determined. The spectrum contains both perpendicular and parallel rotational sub-band structures. The latter are partially due to axis-switching in the transition and Coriolis coupling in the excited state, but are relatively more intense and show a different J-dependence to that expected on the basis of these mechanisms. Intensity borrowing from a predicted but unobserved shorter wavelength transition explains the observations. We have calculated the energy and structure of HCBr in the state at the MRCI/cc-pVTZ level and find . The band system should be experimentally observable. [Copyright &y& Elsevier]

New spectra of methylene, CH[sub 2] , in the near infrared region of the singlet band system have been obtained. They were detected by laser transient absorption spectroscopy following excimer laser flash photolysis of ketene, CH[sub 2] CO, at 308 nm. The new data provide information on the nature and energies of rotational levels in three vibronic states that lie between approximately 11 500 and 13 000 cm[sup –1] above the zero point level of the lower, ã[sup 1] A[sub 1] , state. Taken together with previous measurements, a fairly complete picture of the vibronic energy levels of CH[sub 2] in this region can be built up. There is generally good agreement with the recent computational studies on the singlet states of CH[sub 2] , but some evidence for discrepancies for levels with K = 3 and higher near the minimum of the [sup 1] B[sub 1] potential surface. PACS No.: 33.20EaNous avons trouvé de nouveaux spectres du méthylène CH[sub 2] dans la région du proche infrarouge de la bande du singulet. Nous les avons observés par spectroscopie laser rapide suivant la photolyse du CH[sub 2] CO par impulsion d'un laser excimère à 308 nm. Ces nouvelles données nous renseignent sur la nature et l'énergie des niveaux rotationnels dans trois états vibroniques se trouvant approximativement entre 11 500 et 13 000 cm[sup –1] au dessus du point zéro de l'état ã[sup 1] A[sub 1] . Considérés avec des résultats déjà obtenus, ils nous fournissent une image assez complète des niveaux vibroniques du CH[sub 2] . L'accord avec les récentes études numériques sur les états singulets est très bon, mais des différences apparaissent pour les niveaux avec K = 3 près du minimum de la surface de potentiel [sup 1] B[sub 1] .[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH2 (ã 1A1) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH2 is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH2 population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Detection of OH and HO 2 has been demonstrated using FM laser absorption spectroscopy. • Room temperature photolysis of H 2 O 2 at 193 nm in argon has been used as a test system. • The FM sensitivity is estimated to be on the order of 1011 molecule cm−3. • The product branching measured here is in excellent agreement with previous work. Abstract The photodissociation of H 2 O 2 dilute in argon at 193 nm has been investigated using near infrared frequency modulation laser absorption spectroscopy on selected rotational lines of the OH vibrational overtone bands between 6500 and 7100 cm−1 to detect the OH and HO 2 radical products with the same laser. Sequential OH and HO 2 time-dependent profiles have been measured at room temperature and total pressures ranging from 3 to 56 Torr. Kinetic analysis gives the product distribution following H 2 O 2 photolysis of (82 ± 1) % OH and (18 ± 1) % HO 2 under the present conditions, in excellent agreement with previously reported measurements. [ABSTRACT FROM AUTHOR]

Abstract: We report measurements of self- and nitrogen-pressure broadening of the P(11) line in the ν 1 + ν 3 combination band of acetylene at 195 739.649 5135(80)GHz by absorption of radiation emitted by an extended cavity diode laser referenced to a femtosecond frequency comb. Broadening, shift and narrowing parameters were determined at 296K. For the most appropriate, hard collision, model in units of cm−1/atm, we find 0.146317(27), 0.047271(104) and −0.0070819(22) for the acetylene self-broadening, narrowing and shift, and 0.081129(35), 0.022940(74) and −0.0088913(25) respectively, for the nitrogen-broadening parameters. The uncertainties are expressed as one standard deviation (in parenthesis) in units of the last digit reported. These parameters are 2–3 orders of magnitude more precise than those reported in previous measurements. Similar analyses of the experimental data using soft collision and simple Voigt lineshape models were made for comparison. [Copyright &y& Elsevier]