Line strengths and transition dipole moment of the nu2 fundamental band of the methyl radical

The line strengths of nine Q-branch lines in the nu(2) fundamental band of the methyl radical in its ground electronic state have been measured by diode laser absorption spectroscopy. The vibration-rotation spectrum of methyl was recorded in a microwave discharge in ditertiary butyl peroxide heavily diluted in argon. The absolute concentration of the radical was determined by measuring its kinetic decay when the discharge was extinguished. The translational, rotational, and vibrational temperatures, also required to relate the line strengths to the transition dipole moment, were determined from relative integrated line intensities and from the Doppler widths of the lines after allowing for instrumental factors. The line strengths of the nine Q-branch lines were used to derive a more accurate value of the transition dipole moment of this band, mu(2)=0.215(25) D. Improved accuracy over earlier measurements of mu(2) (derived from line strengths of single lines) was obtained by integrating over the complete line profile instead of measuring the peak absorption and assuming a Doppler linewidth to deduce the concentration. In addition, a more precise value for the rate constant for methyl radical recombination than available earlier was employed. The new value of mu(2) is in very good agreement with high-quality ab initio calculations. Furthermore, the ratio of the transition dipole moments of the nu(2) and nu(3) fundamental bands in the gas phase is now in highly satisfactory agreement with the ratio determined for the condensed phase.     

Infrared diode laser spectroscopy of the deltanu=2 band of AlF

A vibrational-rotational spectrum of the deltanu=2 transitions of a high-temperature molecule AlF was observed between 1,490 and 1,586 cm(-1) with a diode laser spectrometer. Measurements were made on the nu=3-1, 4-2, 5-3 and 8-6 bands at a temperature of 900 degrees C. Measured spectral lines were fitted to effective band constants nu(0), B(nu) and D(nu) for each band. Present measurements were made with only one Pb-salt laser diode. Physical significance of the effective band constants is discussed.     

Determination of the dipole moment of thioformaldehyde in the a3A2 excited state by laser phosphorescence excitation spectroscopy

Doppler-limited phosphorescence excitation spectra have been recorded at various electric fields for two rotational transitions in the $\text{a}$³A₂-$\text{X}$¹A₁ 0-0 band of H₂CS. Stark splittings were resolved, and were used to determine the dipole moment in the excited electronic state. The value found, 0.57(3) D, is of the order expected by comparison with dipole moments determined for other states of H₂CS, but rather lower than that predicted by ab initio calculations.     

Measurement of the sixth overtone band of nitric oxide, and its dipole moment function, using cavity-enhanced frequency modulation spectroscopy

We applied cavity-enhanced frequency modulation absorption spectroscopy (also known as noise-immune cavity-enhanced optical heterodyne molecular spectroscopy) to perform high-resolution spectroscopy of the sixth overtone band of nitric oxide near 797 nm. By using novel high-bandwidth balanced detectors, baseline variations caused by residual amplitude modulation were significantly reduced. The ultrahigh sensitivity (2 x 10(-10) cm(-1) minimum detectable absorption at 1 Hz detection bandwidth) of our spectrometer allowed accurate measurements of 15 individual line intensities of P- and R-branch transitions in the 2Pi(1/2)-2Pi(1/2) subband. A vibrational transition moment of 3.09(6) muD+/-13% and Herman-Wallis coefficients of a = -0.0078(26) and b = 0.001 25(45) were found by fitting the line intensities. Based on our measured transition moment, and those of other transitions from the literature, a new parametrization for the electric dipole moment function (EDMF) of nitric oxide, valid for 0.91 < or = r < or = 1.74 A, has been extracted. The residuals of this fit demonstrate that the derived EDMF is the most accurate representation to date of the dipole moment function. The new EDMF will be valuable for accurate intensity prediction of transitions that cannot be easily measured experimentally.     

Measurements of line strengths in the 2nu1 band of the HO2 radical using laser photolysis/continuous wave cavity ring-down spectroscopy (cw-CRDS)

Absolute absorption cross sections of the absorption spectrum of the 2nu1 band of the HO2 radical in the near-IR region were measured by continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to laser photolysis in the wavelength range 6604-6696 cm(-1) with a resolution better than 0.003 cm(-1). Absolute absorption cross sections were obtained by measuring the decay of the HO2 self-reaction, and they are given for the 100 most intense lines. The most important absorption feature in this wavelength range was found at 6638.20 cm(-1), exhibiting an absorption cross section of sigma = 2.72 x 10(-19) cm2 at 50 Torr He. Using this absorption line, we obtain a detection limit for the HO2 radical at 50 Torr of 6.5 x 10(10) cm(-3).     

Determination of the dipole moment of thioformaldehyde in the A2 excited state by laser phosphorescence excitation spectroscopy

Doppler-limited phosphorescence excitation spectra have been recorded at various electric fields for two rotational transitions in the ³A₂- ¹A₁ 0-0 band of H₂CS. Stark splittings were resolved, and were used to determine the dipole moment in the excited electronic state. The value found, 0.57(3) D, is of the order expected by comparison with dipole moments determined for other states of H₂CS, but rather lower than that predicted by ab initio calculations.     

Infrared laser spectroscopy of jet-cooled carbon clusters: the nu 5 band of linear C9

The nu 5 antisymmetric stretching vibration of 1 sigma+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079-2081 cm-1 were assigned to this band. A combined least squares fit of these transitions with previously reported nu 6 transitions yielded the following molecular constants for the nu 5 band: nu 0 = 2 079.673 58(17) cm-1, B"= 0.014 321 4(10) cm-1, and B'=0.014 288 9(10) cm-1. The IR intensity of the nu 5 band relative to nu 6 was found to be 0.108 +/- 0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD).     

Characterization of silicon-carbon clusters by infrared laser spectroscopy: the nu 3(sigma u) band of linear Si2C3

The nu 3(sigma u) fundamental vibration of 1 sigma g+ Si2C3 has been observed using a laser vaporization-supersonic cluster beam-diode laser spectrometer. Forty rovibrational transitions were measured in the range of 1965.8 to 1970.9 cm-1 with a rotational temperature of 10-15 K. A least-squares fit of these transitions yielded the following molecular constants: nu 3(sigma u)=1968.188 31(18) cm-1, B"=0.031 575 1(60) cm-1, and B'=0.031 437 4(57) cm-1. These results are in excellent agreement with recent Fourier transform infrared (FTIR) measurements of Si2C3 trapped in a solid Ar matrix [J. Chem. Phys. 100, 181(1994)] and with ab initio calculations [J. Chem. Phys. 100, 175 (1994)] which suggest cumulenic-like bonding for Si2C3, analogous to the isovalent C5 carbon cluster.     

Diode laser spectroscopy of the nu(8) band of the SF(5)Cl molecule

Diode laser spectra of SF(5)Cl have been recorded in the nu(8) band region at a temperature of ca. 240 K, a pressure of 0.25 mbar and an instrumental bandwidth of ca. 0.001 cm(-1). Four regions have been studied: a first one in the P-branch (906.849-907.687 cm(-1)), a second one in the Q-branch (910.407-910.944 cm(-1)), and two other ones in the R-branch (913.957-914.556 and 917.853-918.705 cm(-1) ). The whole nu(1)/nu(8) dyad of SF(5)35Cl has been previously recorded in the group of Professor H. Burger in Wuppertal, thanks to a Fourier transform infrared spectrometer. These data have thus been combined with our diode laser ones in the aim of refining the analysis. We used an effective Hamiltonian developed up to the fourth order and a set of programs called C(4nu)TDS. One thousand three hundred and forty-six transitions for nu(1), 495 (FTIR: 351; diode laser: 144) transitions for nu(8), and 406 ground state combination differences have been assigned and fitted. A global fit has been obtained with a rms of 0.00081 cm(-1) for the nu(1) band, 0.0012 cm(-1) for the FTIR data of the nu(8) band, 0.00055 cm(-1) for the diode laser data of this same band, and 0.00064 cm(-1) for the ground state. It appears that more data (for instance, using a supersonic jet) are still necessary to obtain a completely satisfactory analysis of the nu(8) region.     

Energetics and dipole moment of transition metal monoxides by quantum Monte Carlo

The transition metal (TM) oxygen bond appears very prominently throughout chemistry and solid-state physics. Many materials, from biomolecules to ferroelectrics to the components of supernova remnants, contain this bond in some form. Many of these materials' properties depend strongly on fine details of the TM-O bond, which makes accurate calculations of their properties very challenging. Here the authors report on highly accurate first principles calculations of the properties of TM monoxide molecules within fixed-node diffusion Monte Carlo and reptation Monte Carlo.     

The vibrationless A<--X transition of the jet-cooled deuterated methyl peroxy radical CD3O2 by cavity ringdown spectroscopy

The nearly rotationally resolved spectrum of the A (2)A(')<--X (2)A(") 0(0)(0) transition of perdeutero methyl peroxy near 1.35 microm has been studied via pulsed cavity ringdown spectroscopy. Albeit, this is a weak transition, it is possible to observe the spectrum under jet-cooled conditions (approximately 15 K) by combining a source of narrow-bandwidth radiation (approximately 250 MHz) with a supersonic slit-jet expansion incorporating an electric discharge. The near infrared radiation was obtained by using stimulated Raman scattering and a pulsed, nearly Fourier-transform-limited Ti:sapphire amplifier seeded by a scanable cw Ti:sapphire ring laser. The experimental spectrum has been fitted using a model Hamiltonian that includes the rigid body rotation of an asymmetric top and the spin-rotation interaction. An excellent quality fit was obtained resulting in the determination of 15 molecular parameters characterizing the A and X states. Other results reported for CD(3)O(2) include an estimate of the radical concentration and the vibronic transition dipole from the observed absorption intensities. Details about the spectral linewidths are also discussed.     

High resolution spectroscopy of the nu(3) band of DCOOD

The rotation-vibration spectrum of DCOOD has been recorded in the carbonyl stretch (nu(3)) region. Using a standard S-reduced Watson Hamiltonian in the I(r) representation, 225 lines could be fitted to a vibrational-rotational band. A full set of molecular constants was obtained. The nu(3) band is found to be strongly perturbed in the K(a): 1<--1 and K(a): 2<--2 subband. The perturbation is attributed to a Fermi resonance with the 2nu(8) overtone band and Coriolis coupling to a combination band (nu(4)+nu(7)). The band center is determined to be 1725.1218(1) cm(-1) which is more than 10 cm(-1) shifted compared to previous studies.     

Measurement of the second moment of a Raman band

A method for determining the second moment of a Raman band has been proposed. The band shape is assumed to be expressed in terms of a mathematical function, which is derived from the Fourier transform of the Kubo correlation function. Parameters in the expression are adjusted with the least squares method to fit the experimental band shape to the theoretical expression. The second moment can be obtained from those parameters determined. The advantage of the proposed method lies in that the second moment can be determined even for overlapped bands.     

Molecular fine structure and transition dipole moment of NO2 using an external cavity quantum cascade laser

Rotational vibrational fine structure and transition dipole moment of NO₂ is measured using Doppler free saturation spectroscopy with an external grating cavity quantum cascade laser (QCL). The QCL wavelength is calibrated using a 310 cm long internally coupled Fabry–Perot interferometer. We obtain a frequency splitting of 139.68 ± 0.06 MHz (0.0047 cm⁻¹) between the spin doublets (17) of 000 → 001 transition of NO₂. The resolution of the QCL based saturation spectrometer is limited by the QCL linewidth of 3.99 MHz ( 0.00013 cm⁻¹) deduced from the half width of the Lamb dips. The Lamb dip spectroscopy is utilized to obtain a vibrational dipole moment of 0.37 Debye for the (17) transitions.     

Conformational identification and dipole moment of 4-fluoro-1-butene by microwave spectroscopy

The microwave spectrum of 4-fluoro-1-butene has been recorded in the frequency region 26.5–40.0 GHz. A-type rotational transitions in the ground vibrational state have been assigned. The effective rotational constants and centrifugal distortion constants were determined to be A = 19 196 ± 323, B = 2132.48 ± 0.01, C = 2112.52 ± 0.01 MHz, D_J = 0.70 ± 0.03 and D_JK = −26.16 ± 0.05 kHz. Analysis of the measured Stark effects gave a dipole moment of 1.80 D with components of |u_a| = 1.62 ± 0.01, |u_b| = 0.68 ± 0.05 and |u_c| = 0.39 ± 0.14 D. From the experimental rotational constants and dipole moments, it is suggested that the assigned spectra have resulted from the skew—trans conformer.     

Three-dimensional orientation of the Qy electronic transition dipole moment within the chlorophyll a molecule determined by femtosecond polarization resolved VIS pump-IR probe spectroscopy

Chlorophyll a (Chl a) is the most abundant pigment on earth. In all plants, algae, and cyanobacteria, it plays a pivotal role as an antenna and reaction center pigment in the primary steps of photosynthesis. In the past, a true three-dimensional (3D) experimental determination of the Qy electronic transition dipole moment orientation could not be obtained. With combined femtosecond polarization resolved VIS pump-IR probe experiments and theoretical calculations of the infrared transition dipole moments (tdm's) in the electronic ground state, we determined the 3D orientation of the Qy electronic tdm of Chl a within the molecular structure. Polarization resolved experiments provided angles of the Qy electronic tdm with three different infrared tdm's, whose orientations within the molecular structure were taken from our theoretical calculations. The orientation of the Qy tdm results from the intersection of all three angles and was found to have an angle of (78 +/- 3)degrees with the x-axis, (12 3)degrees with the y-axis, and (86 +/- 2)degrees with the z-axis.     

Continued fraction representations of the dipole moment function and the electronic transition moment function of diatomic molecules

Explicit consideration of the X² Π state of OH and the X¹Σ⁺ state of HF is used to demonstrate that continued fraction approximations provide particularly good representations to the dipole moment functions and allow for extrapolation outside the region for which data is available. Continued fractions also prove useful for representing other molecular properties which are functions of the internuclear separation. As a second example a study of the B¹Σ_u^÷ ? X¹Σ_g⁺ electronic transition of H₂ is included.     

Spectroscopic investigation of magnetic dipole allowed transitions through the magnetic transition moment

An experimental technique for detecting magnetic dipole allowed transitions by their dispersion-induced circular dichroism (DICD) is discussed. The DICD spectra of three nitrogen heterocycles (pyridine, pyrazine and tetraphenylporphyrin) are shown to be dominated by the n — π^* transitions, as distinct from the normal absorption spectra which are dominated by the electric dipole allowed π — π^* bands.     

Conformational analysis of organophosphorus compounds by means of IR spectroscopy and dipole moment methods

Some aspects of rotational isomerism about an ordinary P—X bond in compounds

have been discussed on the basis of spectroscopic data. Changes in spectra in the process of phase transition with variation of dielectric permeability of the environment and temperature have been analyzed. A scheme of structure identification by the dipole moment method with determination of conformer concentration from IR-spectra has been proposed. Data from IR-spectra, dipole moments and the Kerr effect have been applied to complicated systems.Gauche- and trans-orientated O—C and PO bonds are characteristic of compounds for which X = O. It has been shown that in the case of rotation of a chloromethyl group at the phosphorus atom trans and gauche arrangements of C—Cl and PO (S) are stable. The position of the conformational equilibrium depends on the nature of the substituents. The important role of electrostatic interaction has been established.     

Study of radical methyl methacrylate-methacrylic acid copolymerization in isopropyl alcohol by dynamic laser scattering and C-NMR spectroscopy

The copolymerization of methyl methacrylate and methacrylic acid in isopropyl alcohol is studied by kinetic, dynamic laser scattering and ¹³C-NMR methods. Correlations are established between the dependence of the copolymerization rate, the apparent average molecular weight, the diffusion coefficients and the configurational triad composition on the monomer feed. These correlations and the fact that both copolymerization constants are smaller than unity (r_MMA = 0.63 ± 0.03; r_MA = 0.25 ± 0.05) are in good agreement with the assumption that a comonomer complex takes part in the propagation reaction. A new Markov chain method for the estimation of the configurational triad mole fraction which accounts for the complex participation in a macromolecule formation is developed. Qualitative criteria for evidence the participation of the comonomer complex in copolymerization are proposed using experimental and terminal model calculated mole fractions of the compositional triads and diads.