Asymmetric Top Molecules Research Articles

What can be learned about molecular reorientation from single molecule polarization microscopy?

We have developed a general approach for the calculation of the single molecule polarization correlation function C(t), which delivers a correlation of the emission dichroisms at time 0 and t. The approach is model independent and valid for general asymmetric top molecules. The key dynamic quantities of our analysis are the even-rank orientational correlation functions, the weighted sum of which yields C(t). We have demonstrated that the use of nonorthogonal schemes for the detection of the single molecule polarization responses makes it possible to manipulate the weighting coefficients in the expansion of C(t). Thus valuable information about the orientational correlation functions of the rank higher than the second can be extracted from C(t).

Laser-induced circular dichroism by a resonant circularly polarized pump optical field

The paper presents and discusses a generalization of the equations of laser-induced circular dichroism (LICD) derived for linear molecules. LICD for freely rotating molecules by a resonant circularly-polarized pump field is an optical analogue of the Faraday effect in isotropic media. Its fundamental formulae and existent conditions have been deduced using the density matrix theory with a two-level model and time-reversal arguments. The antisymmetric rotational polarizability and LICD of symmetric and asymmetric top molecules are studied concretely using perturbation theory and angular momentum algebra. The order of magnitude of LICD and the physical phenomena closely related with it have been discussed. The results show that the LICD effect is exhibited by all freely rotating molecules and may provide useful information on optical activity and antisymmetric polarizability.

Rotational excitation of water by hydrogen molecules: Comparison of results from classical and quantum mechanics

Quasiclassical trajectory calculations are carried out for rotational excitation of water by hydrogen molecules. State-to-state rate coefficients are determined at 100 K and are compared to available quantum results. A good agreement between classical and quantum rates is observed for downward transitions, with an average accuracy of classical results better than a factor of 2. It is thus found that the ambiguities described by Faure and Wiesenfeld [J. Chem. Phys. 121, 6771 (2004)] can be solved in the particular case of waterlike asymmetric-top molecules.

Gas phase electronic spectrum of propadienylidene C 3H 2

The rotationally resolved vibronic bands in the forbidden A ˜ 1 A 2 – X ˜ 1 A 1 electronic transition of the cumulene carbene C 3H 2 have been observed in the gas phase by cavity ring down absorption spectroscopy through a supersonic planar plasma with allene as precursor. The band detected in the 16 223 cm −1 region is a result of vibronic interaction and is assigned to a combination of a 1 and b 2 vibrations with a frequency around 2250 cm −1. Another vibronic band near 15 810 cm −1 has an unusual rotational structure because the K a = 0–1 subband is absent. It is assigned to a combination of a 1 and b 1 vibrations, ∼1850 cm −1, which borrow intensity from the near lying B ˜ 1 B 1 state due to a-type Coriolis coupling. A rotational analysis using a conventional Hamiltonian for an asymmetric top molecule yields molecular constants for the vibrational excited levels of the à 1 A 2 state, which were used for the determination of the geometry. The stronger B ˜ 1 B 1 – X ˜ 1 A 1 transition of C 3H 2, measured in a neon matrix in the 16 161–24 802 cm −1 range, was not detected. The reason for this is a short lifetime of the B ˜ state, leading to line broadening.

Alignment enhancement by the combination of a short and a long laser pulse

The alignment dynamics of polyatomic molecules subject to two temporally overlapping nonresonant laser pulses is studied theoretically and experimentally. We examine the potential advantage of combining a long (compared to the molecular rotational periods) with a short pulse, where both laser fields are linearly polarized with common polarization direction. Experimentally, iodobenzene molecules are irradiated by a $2\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ pulse at $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ synchronized to the peak of a $9\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ pulse at $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The alignment dynamics following the short laser pulse is measured using time-resolved Coulomb explosion with a delayed $25\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$-long intense laser pulse. The alignment attained with the two-pulse combination is significantly enhanced compared to that attainable with either a short or a long pulse alone under nonionizing conditions. Numerically, we solve the time dependent Schr\odinger equation nonperturbatively for asymmetric top, symmetric top, and linear molecules subject to similar combinations of long and short excitation pulses. The alignment dynamics, and in particular the alignment enhancement, are explained and their generality is tested.

Computer Assisted Assignments of Rotationally Resolved Molecular Spectra

A graphically oriented interactive program for assignments of rotationally resolved molecular spectra has been devised. The program functions by grouping spectral lines in term of the second difference principle, and graphing spectral intensity versus frequency in a bar graph of the selected groups, distinguished by color and/or line-type. This allows for easy detection of regular patterns buried in the observed spectrum. Furthermore, it includes a LoomisWood view for assisting in spectral assignments. As an example, the program was applied in assigning the molecular spectrum of the production in the discharge of PCl3 buffered by helium gas, which may belong to several species. The results suggest that the program is highly efficient and quite useful in the assignment and the analysis of molecular spectra, especially those of symmetric top, slightly asymmetric top and linear molecules. The accuracy and efficiency of this program will likely ensure its wide application in the processing of molecular spectra.

High resolution FTIR study of the ν5 and ν6 bands of CH2D35Cl: analysis of resonances and determination of ground and upper state constants

The isotopically pure form of methyl chloride, CH2D35Cl, was synthesized and investigated by Fourier transform infrared spectroscopy with an unapodized resolution of 0.004 cm−1 in the range 650–900 cm−1, the region of the lowest fundamentals ν5 (827 cm−1) and ν6 (714 cm−1). These distinct bands have been analysed in detail in the P-, Q- and R-branches. In spite of their expected a/b-hybrid nature, both envelopes show the peculiar characteristic of only a-type bands of near prolate asymmetric top molecules. Ground state parameters have been determined for the first time through ground state combination differences from both bands. Parameters of the excited vibrational states and coupling constants have been obtained using a model which accounts for c-type Coriolis interaction and ΔKa = ± 2 anharmonic resonance.

Electron-impact study of formaldehyde using the R-matrix method

The R-matrix method, involving eight target states in the close-coupling formalism, is used to calculate elastic integral, differential and momentum transfer cross sections for electron impact on the formaldehyde molecule. We have also obtained the excitation cross sections for the seven lowest-lying electronically excited states which have symmetries 1 3A1, 1 1,3A2, 1 1,3B1 and 1 1,3B2. Their vertical excitation energies from the equilibrium geometry of the ground state X 1A1 lie in the range 3.58–9.99 eV. Configuration interaction (CI) wavefunctions are used to represent the target states. In our CI model, we keep the six core electrons frozen in doubly occupied molecular orbitals 1a1, 2a1 and 3a1. The complete active space consists of ten valence electrons that are allowed to move freely among the eight molecular orbitals: 4a1, 5a1, 6a1, 1b1, 2b1, 1b2, 2b2 and 3b2. In this CI model, we obtain good agreement of the dipole moment of the ground state with the experimental value, and a good representation of the vertical excitation spectrum of the excited states included in our calculation. We have also investigated the electron impact rotationally elastic and rotationally excitation transitions for the ground state for this asymmetric top molecule and obtained the rotationally resolved differential and integral cross sections for energies up to 20 eV. Our calculations do not detect any bound H2CO− states at the equilibrium geometry of the H2CO molecule. We find a shape resonance of the 2B1 symmetry with its resonance position at 1.32 eV and a corresponding resonance width of 0.546 eV at the equilibrium geometry of the molecule. This resonance provides a pathway for dissociative electron attachment when the CO bond is stretched beyond 3a0. Born correction is applied for the elastic and the dipole allowed transitions to account for higher partial waves excluded in the R-matrix calculation. We also compare R-matrix differential, partial, momentum transfer and excitation cross sections with the other work.

Observation of a new phosphorus-containing reactive intermediate: Electronic spectroscopy and excited-state dynamics of the HPBr free radical

The A (2)A(')-X (2)A(") electronic spectra of jet-cooled HPBr and DPBr have been obtained for the first time using the pulsed electric discharge technique with a precursor mixture of PBr(3) and H(2)/D(2). Laser-induced fluorescence and single vibronic level emission spectra gave the bending and P-Br stretching frequencies in the ground and excited states of both isotopomers. Rotational analyses of the HPBr and DPBr 0(0) (0) bands showed small spin splittings characteristic of a doublet-doublet transition of an asymmetric-top molecule. From the ground- and excited-state rotational constants, effective (r(0)) structures were derived with r(")(PH)=1.4307(86) A, r(")(PBr)=2.2021(9) A, and theta(")=95.2(8) degrees, and r(')(PH)=1.434(31) A, r(')(PBr)=2.1669(26) A, and theta(')=115.5(16) degrees . In a few favorable cases, further hyperfine splitting of the spin-rotation energy levels has been observed, due to the excited-state Fermi contact interaction of the unpaired electron with the spin magnetic moment of the (31)P nucleus, with a(F) (')=0.064(9) cm(-1) for HPBr. Fluorescence depletion spectroscopy and lifetime measurements indicate that higher vibrational levels of the A (2)A(') state are predissociated by a X (2)A(") dissociative continuum. CCSD(T)/aug-cc-pVTZ calculations predict that the most likely dissociation process is HPBr (X (2)A("))-->PH((3)Sigma(-))+Br((2)P(u)).

Accurate rest frequencies of submillimeter-wave lines of H 2D + and D 2H +

We re-examined the submillimeter-wave transition frequencies of H 2D + ( J = 1 10 − 1 11 at 372.4 GHz) and D 2H + ( J = 1 10 − 1 01 at 691.7 GHz) to resolve suggested slight difference in velocity ( v LSR) of these species detected in the cold pre-stellar core 16293E recently. Both H 2D + and D 2H + were generated in a magnetically confined extended-negative glow discharge of a gaseous mixture of H 2/D 2/Ar. A combination of small improvements in various aspects of the measurements such as double modulation technique combined with a conventional frequency modulation and magnetic field modulation and more efficient signal accumulation method allowed us to improve signal-to-noise ratio, and thus to determine the transition frequencies more accurately. Both transition frequencies for the H 2D + and D 2H + lines have been thus determined to be 372421.385(10) and 691660.483(20) MHz, respectively. These precise rest frequencies suggest that the v LSR of H 2D + and D 2H + in the pre-stellar core 16293E are indeed different as indicated in a recent astronomical observation. In addition, in this investigation, another transition of H 2D + which falls in this frequency region, J = 3 21 − 3 22 transition, has been observed at 646430.293(50) MHz. As H 2D + is a lightest asymmetric-top molecule and it is difficult to predict the rotational transition frequencies by using the effective asymmetric rotor Hamiltonian, any new observation of the rotational lines will be useful to improve the molecular parameters. The molecular constants for the ground state have been obtained for H 2D + and D 2H + by fitting these new measured frequencies together with the combination differences.

Second virial coefficients of asymmetric top molecules

A short self-contained derivation is given for the second virial coefficient B2(T) of a gas consisting of identical interacting asymmetric rigid rotors. The resulting expression is correct through variant Planck's over h2. First, the canonical partition function is derived by means of an variant Planck's over h expansion of exp[-H/(k(B)T)] due to Friedmann [Adv. Chem. Phys. 4, 225 (1962)]. The present work applies angular momentum operators and known facts from angular momentum theory. It is considerably more accessible than Friedmann's exposition, which is not based on angular momentum operators, but instead on explicit derivatives with respect to Euler angles. The partition function obtained from the variant Planck's over h expansion is applied to the derivation of an expression for B2(T) that is identical in appearance to the expression for symmetric rotors of T Pack [J. Chem. Phys. 78, 7217 (1983)]. The final equation in this work is valid for rigid rotors of any symmetry.

Field-Free Three Dimensional Molecular Axis Alignment

We investigate strategies for field-free three dimensional molecular axis alignment using strong nonresonant laser fields under experimentally realistic conditions. Using the polarizabilites and rotational constants of an asymmetric top rotor molecule (ethene, C2H4), we consider three different methods for axis alignment of a Boltzmann distribution of rotors at 4 K. Specifically, we compare the use of impulsive kick laser pulses having both linear and elliptical polarization to the use of elliptically polarized switched laser pulses. We show that an enhanced degree of field-free three dimensional alignment of ground vibronic state molecules obtains from the use of two orthogonally polarized, time-separated laser pulses.

The Radio Spectra of S3and S4

The pure rotational spectra of the polar asymmetric top molecules S3 and S4 have recently been observed in the laboratory for the first time. Approximately 150 transitions of S3 between 10 and 458 GHz and a similar number for S4 between 6 and 271 GHz have been measured. Many transitions of both molecules are accessible with existing telescopes across the entire radio band. The most interesting astronomical lines to 300 GHz are calculated here from the derived spectroscopic constants to an uncertainty of 1 km s-1 or better in equivalent radial velocity, and even at 700 GHz uncertainties are probably worse by only an order of magnitude. Both S3 and S4 are plausible candidates for detection in Galactic molecular sources, the atmosphere of the Jovian moon Io, and comets.

Formula omitted] top data system ( [formula omitted]TDS) software for spectrum simulation of [formula omitted] asymmetric molecules

The D 2 h TDS ( D 2 h Top Data System) program suite has been developed with the aim of studying any rovibrational band or polyad of X 2 Y 4 ( D 2 h ) asymmetric top molecules. It is based on the same principles as similar programs from our group already released for various molecular symmetries ( T d , O h , C 4 v , C 2 v ). We work in the O ( 3 ) ⊃ D 2 h chain and this choice has consequences on the method used to specify the input parameters of the programs for Hamiltonian and transition moment calculations. Two examples concerning the ν 12 and ν 2 bands of the C 2 H 4 molecule are presented. This suite consists of a series of FORTRAN programs called by a script. The whole package is freely accessible through ftp (user anonymous) at jupiter.u-bourgogne.fr or through the World Wide Web at

Missing levels lead to additional lines: the influence of nuclear spin statistics on femtosecond degenerate four wave mixing spectroscopy of polyatomic systems

We present the results of comprehensive experimental and theoretical studies of the effect of nuclear spin statistics on the structure of rotational recurrences in femtosecond degenerate four-wave mixing (fs DFWM) spectra of asymmetric top molecules. New recurrences of J-type with a period of 1/[4(B + C)] are identified in fs DFWM spectra of near-prolate tops SO2 [H. M. Frey, P. Beaud, T. Gerber, B. Mischler, P. P. Radi and A. P. Tzannis, J. Raman Spectrosc., 2000, 31, 71] and NO2 [I. Pastirk, M. Comstock and M. Dantus, Chem. Phys. Lett., 2001, 349, 71]. Our measurements of the asymmetric oblate-like top CH3NO2 revealed new recurrences of A-type with a period of 1/(8A), A, B and C being the rotational constants.The incorporation of nuclear spin statistics into the fs DFWM simulation-fitting code allowed us to successfully reproduce all these new features and the subsequent theoretical analyses uncovered mechanisms of their appearance. The nuclear spin statistics is shown to have a profound effect on fs DFWM spectra of asymmetric top species, notably with several zero spin nuclei.

Vibration-rotation interactions between overtone and combination levels of asymmetric-top molecules: Application to the infrared spectroscopy of formaldehyde and ketene

The conventional vibration-rotation Hamiltonian for an asymmetric-top molecule is rewritten by expanding the elements of the inverse inertial tensor about the equilibrium molecular geometry. The approach allows the identification of terms in the Hamiltonian that couple states differing by two, three, or four vibrational quanta and hence the calculation of dimensioned Coriolis xi coupling coefficients for interacting fundamental, overtone, and combination levels. The matrix elements that result from the application of the expanded Hamiltonian depend upon the harmonic vibrational wave numbers, equilibrium moments of inertia, Coriolis zeta parameters, and the derivatives of the elements of the inertial tensor matrix with respect to each of the normal coordinates. The Coriolis coupling coefficients may be calculated through evaluation of the summations that result from the appropriate terms. The validity of the approach is demonstrated through the calculation of coupling coefficients for interacting levels in formaldehyde and ketene. The uncertainty in the calculated values of the coupling coefficients is typically better than +/-6%, although the values calculated for interactions that involve low-frequency vibrational modes are less reliable. Comparisons are made between the calculated values and experimental results.

Nonadiabatic laser-induced alignment of iodobenzene molecules

Nonadiabatic alignment of an asymmetric top molecule induced by a short, moderately intense laser pulse is studied theoretically and experimentally. Numerically, we solve nonperturbatively the time-dependent Schr\odinger equation for a general asymmetric top molecule subject to a moderately intense laser field, and analyze the dependence of the alignment dynamics on the field strength and on the rotational temperature. Experimentally, we use time-resolved photofragment imaging to measure the time-dependent angular distributions of the spatial orientation of the molecules. Our studies, using iodobenzene as a test molecule, focus on the short-time alignment dynamics, during and after the pulse.

Rotationally resolved electronic spectrum of propadienylidene

The rotationally resolved electronic spectrum of a vibrational band in the A ˜ 1 A 2 – X ˜ 1 A 1 transition of the cumulene carbon chain C 3H 2 was measured in the 625 nm region in a supersonic discharge using cw cavity ring down spectroscopy. The rotational structure was analysed using a conventional Hamiltonian for an asymmetric top molecule, and the constants A′, B′, and C′ in the upper state were determined. The observed band is assigned to a combination of a 1 and b 2 vibrations with the frequency around 2000 cm −1. The geometries in the 1 A 1, 1 A 2, 1 B 1 states and the intersection point between the latter two were obtained using ab initio calculations. The effective structure in the measured vibrational level of the 1 A 2 state was inferred from the determined constants.

Pendular-state spectroscopy of the S1–S electronic transition of 9-cyanoanthracene

Fluorescence excitation spectra of the S(1)-S(0) origin band of 9-cyanoanthracene have been observed under a uniform electric field up to 200 kV/cm to explore pendular-state spectrum of an asymmetric-top molecule close to the strong field limit. The observed spectra exhibit distinct evolution of the band contour as a function of the applied electric field, which are much different from each other for different excitation configurations. An approximate method suitable for spectrum simulations of large asymmetric-top molecules in a pendular condition is developed for the analysis of the experimental results. The comparison of the observed and simulated spectra shows that the spectra are well ascribed in terms of the pendular-state selection rules, which have recently been derived from theoretical consideration of the pendular-limit representation of energy levels and spectra.

Formula omitted] top data system ( [formula omitted] TDS) software for infrared spectrum simulation of [formula omitted] asymmetric molecules: some improvements to the TDS packages

The spherical top data system (STDS) program suite developed in Dijon has been extended with the aim of studying any rovibrational band or polyad of XY 2 Z 2 ( C 2 v ) asymmetric top molecules. We work in the O ( 3 ) ⊃ T d ⊃ C 2 v chain because these species result from the substitution of two ligands of a corresponding “parent” spherical top and thus are relatively close to tetrahedral symmetry. The choice of this group chain has consequences on the method used to specify the input parameters of the programs for Hamiltonian and transition moment model calculations. One example concerning the ground state of the SO 2F 2 quasi-spherical molecule is presented. As before, this suite consists in a series of FORTRAN programs called by a script. The whole package is freely accessible through ftp (user anonymous) at jupiter.u-bourgogne.fr/dist or through the World Wide Web at http://www.u-bourgogne.fr/LPUB/c2vTDS.html . Further general improvements have been brought about for all the TDS packages (STDS, HTDS, C 4 v TDS) developed in our group and are briefly discussed in the present paper.