Rovibrational spectra of LiH2+, LiHD+ and LiD2+ determined from FCI property surfaces

Full configuration interaction (FCI) has been used in conjunction with the lithium [6s5p3d1f] (Iron, M. A.; et al. Mol. Phys. 2004, 101, 1345) and hydrogen aug-cc-pVTZ basis sets to construct an 83-point potential energy surface of the 1A1 ground state of 7LiH2+. Vibrational and rovibrational wave functions of the (6,7)LiH2+, (6,7)LiHD+, and (6,7)LiD2+ ground states were calculated variationally using an Eckart-Watson Hamiltonian. For (7)LiD2+, rovibrational transition frequencies for K = 0, 1, 2 and J < or = 10 are within ca. 0.1% of recent experimental values (Thompson, C. D.; et al. J. Chem. Phys. 2006, 125, 044310). A 47-point FCI dipole moment surface was embedded in the rovibrational Hamiltonian to calculate vibrational and rovibrational radiative properties. At 296 K, with v < or = 4 and J < or = 4, the 2(02) <-- 3(03) rotational transition in the |001> band was found to have the greatest spectral intensity with respect to the ground electronic states of (6,7)LiH2+, (6,7)LiHD+, and (6,7)LiD2+. In each case, the most intense rovibrational transitions have been assigned unequivocally using the J, Ka, Kc assignment scheme.     

Radiative transition probabilities, lifetimes and dipole moments for the vibrational levels of the X1Sigma+ ground state of 39K85Rb

Using a potential energy curve (based primarily on the RKR potential of Amiot and Verges [J. Chem. Phys. 112, 7068 (2000)]) and a dipole moment function (based primarily on ab initio calculations of Park et al. [Chem. Phys. 257, 135 (2000)]), we have calculated radiative transition probabilities (Einstein A coefficients), radiative lifetimes, and dipole moment expectation values involving all vibrational levels (for several rotational quantum numbers) of the X1Sigma+ ground state of 39K85Rb. We observe that the radiative lifetimes of vibrationally excited levels, in particular, are approximately 10(3)-10(6) seconds, far too long to be significant in most ultracold experiments involving 39K85Rb or its isotopomers. Comparison with other molecules (LiH and HF) suggests that simple scaling (A approximately mu2nu3 approximately tau(-1)) will predict similarly long lifetimes for many other heteronuclear molecules, e.g., RbCs.     

Hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2)

The hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2) have been resolved with sub-Doppler continuous wave perturbation facilitated optical-optical double resonance spectroscopy via A (1)Sigma(u) (+) approximately b (3)Pi(u) mixed intermediate levels. The hyperfine patterns of these three states are similar. The hyperfine splittings of the low rotational levels are all very close to the case b(betaS) limit. As the rotational quantum number increases, the hyperfine splittings become more complicated and the coupling cases become intermediate between cases b(betaS) and b(beta J) due to spin-rotation interaction. We present a detailed analysis of the hyperfine structures of these three (3)Sigma(g) (+) states, employing both case b(betaS) and b(beta J) coupling basis sets. The results show that the hyperfine splittings of the (3)Sigma(g) (+) states are mainly due to the Fermi-contact interaction. The Fermi contact constants for the two d sigma Rydberg states, the 2 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+), are 245+/-5 MHz and 225+/-5 MHz, respectively, while the Fermi contact constant of the s sigma 3 (3)Sigma(g) (+) Rydberg state is 210+/-5 MHz. The diagonal spin-spin and spin-rotation constants, and nuclear spin-electronic spin dipolar interaction parameters of the 3 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+) states are also obtained.     

The B 1sigma+ and X 1sigma+ electronic states of hydrogen fluoride: a direct potential fit analysis

All literature pure rotational and vibration-rotational spectroscopic data on the ground X (1)Sigma(+) electronic state of HF and DF, together with the entire set of spectroscopic line positions from analyses of the B (1)Sigma(+) --> X (1)Sigma(+) emission band systems of HF and DF, have been used in a global least-squares fit to the radial Hamiltonian operators, in compact analytic form, for both electronic states. With a data set consisting of 6157 spectroscopic line positions, the reduced standard deviation of the fit was sigma = 1.028. Sets of quantum mechanically significant rotational and centrifugal distortion constants were calculated for both electronic states using Rayleigh-Schr?dinger perturbation theory.     

Inelastic neutron scattering study of hydrogen in d(8)-THFD(2)O ice clathrate

In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deutrated tetrahydrofuran-water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J=0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned since there was residual orthohydrogen at low temperatures (slow conversion to the ground state) resulting in an observable J=1-->0 transition at 5 K (kT=0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV is interpreted as J=1-->2 transitions. In addition to the transitions between rotational states, there are a series of peaks that arise from transitions between center-of-mass translational quantum states of the confined hydrogen molecule. A band at approximately 9 meV can be unequivocally interpreted as a transition between translational states, while broad features at 20, 25, 35, and 50-60 meV are also interpreted to as transitions between translational quantum states. A detailed comparison is made with a recent five-dimensional quantum treatment of hydrogen in the smaller dodecahedral cage in the SII ice-clathrate structure. Although there is broad agreement regarding the features such as the splitting of the J=1 degeneracy, the magnitude of the external potential is overestimated. The numerous transitions between translational states predicted by this model are in poor agreement with the experimental data. Comparisons are also made with three simple exactly solved models, namely, a particle in a box, a particle in a sphere, and a particle on the surface of a sphere. Again, there are too many predicted features by the first two models, but there is reasonable agreement with the particle on a sphere model. This is consistent with published quantum chemistry results for hydrogen in the dodecahedral 5(12) cage, where the center of the cage is found to be energetically unfavorable, resulting in a shell-like confinement for the hydrogen molecule wave function. These results demonstrate that translational quantum effects are very significant and a classical treatment of the hydrogen molecule dynamics is inappropriate under such conditions.     

Two-color resonant four-wave mixing spectroscopy of highly predissociated levels in the A 2A1 state of CH3S

We report results of two-color resonant four-wave mixing experiments on highly predissociated levels of the methylthio (or thiomethoxy) radical CH3S in its first excited electronic state A 2A1. Following photolysis of jet-cooled dimethyl disulfide at 248 nm, the spectra were measured with a hole-burning scheme in which the probe laser excited specific rotational transitions in band 3(3). The spectral simplification afforded by the two-color method allows accurate determination of line positions and homogeneous linewidths, which are reported for the C-S stretching states 3v(v=3-7) and combination states 1(1)3v(v=0-2), 2(1)3v(v=3-6), and 1(1)2(1)3v(v=0,1) involving the symmetric CH3 stretching (nu1) mode and the CH3 umbrella (nu2) mode. The spectra show pronounced mode specificity, as the homogeneous linewidth of levels with similar energies varies up to two orders of magnitude; nu3 is clearly a promoting mode for dissociation. Derived vibrational wave numbers omega1', omega2', and omega3' of the A state agree satisfactorily with ab initio predictions.     

Fermi interaction between the nu1 and the nu2+4 nu(s) bands of Ar...DN2(+)

Two rotationally fully resolved vibrational bands have been assigned unambiguously to the linear deuteron bound Ar...DN(2) (+) complex by using ground state combination differences. The ionic complex is formed in a supersonic planar plasma expansion optimized and controlled by a mass spectrometer and is detected in direct absorption using tunable diode lasers and applying production modulation spectroscopy. The band origins are located at 2436.272 cm(-1) and at 2435.932 cm(-1) and correspond to the nu(1) band (NN stretch) and to the nu(2)+4 nu(s) combination band (DN and intermolecular stretch), respectively. The two bands overlap strongly and the large intensity of the combination band is explained in terms of a Fermi interaction. This interaction perturbs the observed transitions, particularly for low J values. Least-squares fitting yields values for the Fermi interaction parameters of F(0)=0.332 cm(-1) and F(J)=-0.001 46 cm(-1) and results in accurate rotational constants. These are discussed both from an experimental and a theoretical point of view.     

Contribution of water dimer absorption to the millimeter and far infrared atmospheric water continuum

We present a rigorous calculation of the contribution of water dimers to the absorption coefficient alpha(nu,T) in the millimeter and far infrared domains, over a wide range (276-310 K) of temperatures. This calculation relies on the explicit consideration of all possible transitions within the entire rovibrational bound state manifold of the dimer. The water dimer is described by the flexible 12-dimensional potential energy surface previously fitted to far IR transitions [C. Leforestier et al., J. Chem. Phys. 117, 8710 (2002)], and which was recently further validated by the good agreement obtained for the calculated equilibrium constant Kp(T) with experimental data [Y. Scribano et al., J. Phys. Chem. A. 110, 5411 (2006)]. Transition dipole matrix elements were computed between all rovibrational states up to an excitation energy of 750 cm(-1), and J=K=5 rotational quantum numbers. It was shown by explicit calculations that these matrix elements could be extrapolated to much higher J values (J=30). Transitions to vibrational states located higher in energy were obtained from interpolation of computed matrix elements between a set of initial states spanning the 0-750 cm(-1) range and all vibrational states up to the dissociation limit (approximately 1200 cm(-1)). We compare our calculations with available experimental measurements of the water continuum absorption in the considered range. It appears that water dimers account for an important fraction of the observed continuum absorption in the millimeter region (0-10 cm(-1)). As frequency increases, their relative contribution decreases, becoming small (approximately 3%) at the highest frequency considered nu=944 cm(-1).     

Spectroscopy, dissociation dynamics, and potential energy surfaces for CN(A)-Ar

The A (2)Pi-X (2)Sigma(+) band system of CN-Ar has been examined using fluorescence depletion and action spectroscopy techniques. Eight vibronic bands of the complex were observed in association with the monomer 3-0 transition. Pump-probe measurements were used to characterize CN(A (2)Pi(32),nu=3) fragments from direct photodissociation of CN(A (2)Pi,nu=3)-Ar and CN(X (2)Sigma(+),nu=7) fragments from CN(A (2)Pi,nu=3)-Ar predissociation. The latter showed a marked preference for population of positive parity diatomic rotational levels. Bound state calculations were used to assign the A-X bands and to obtain fitted potential energy surfaces for the A state. The average potential obtained from fitting had a well depth of D(e)=137.8 cm(-1). High-level ab initio calculations were used to obtain equilibrium Jacobi coordinates of theta(e)=94 degrees and R(e)=7.25 bohr. The near-symmetric character of the fitted potential energy surface was consistent with the symmetry preference observed in the predissociation dynamics.     

Very accurate potential energy curve of the LiH molecule

We present very accurate calculations of the ground-state potential energy curve (PEC) of the LiH molecule performed with all-electron explicitly correlated Gaussian functions with shifted centers. The PEC is generated with the variational method involving simultaneous optimization of all Gaussians with an approach employing the analytical first derivatives of the energy with respect to the Gaussian nonlinear parameters (i.e., the exponents and the coordinates of the shifts). The LiH internuclear distance is varied between 1.8 and 40 bohrs. The absolute accuracy of the generated PEC is estimated as not exceeding 0.3 cm(-1). The adiabatic corrections for the four LiH isotopologues, i.e., (7)LiH, (6)LiH, (7)LiD, and (6)LiD, are also calculated and added to the LiH PEC. The aforementioned PECs are then used to calculate the vibrational energies for these systems. The maximum difference between the computed and the experimental vibrational transitions is smaller than 0.9 cm(-1). The contribution of the adiabatic correction to the dissociation energy of (7)LiH molecule is 10.7 cm(-1). The magnitude of this correction shows its importance in calculating the LiH spectroscopic constants. As the estimated contribution of the nonadiabatic and relativistic effects to the ground state dissociation energy is around 0.3 cm(-1), their inclusion in the LiH PEC calculation seems to be the next most important contribution to evaluate in order to improve the accuracy achieved in this work.     

Isomeric interconversion in the linear Cl(-)-HD anion complex

The rotationally resolved infrared photodissociation spectrum of Cl(-)-HD is measured in the HD stretch region. Two Sigma-Sigma bands are observed, corresponding to transitions from the ground state [the (nuHD = 0, n = 0) level] and first excited intermolecular bend state [the (nuHD = 0, n = 1) level]. The (nuHD = 0, n = 0) and (nuHD = 0, n = 1) states are predominantly associated with the linear Cl-...DH and Cl-...HD geometries, respectively. The spectrum is complicated by perturbative interactions between levels of the (nuHD = 0, n = 0) and (nuHD = 0, n = 1) rotational manifolds and between levels of the (nuHD = 1, n = 0) and (nuHD = 1, n = 1) rotational manifolds. A global fit to the transition frequencies, taking the lower and upper state perturbations into account, yields zero-order rotational and centrifugal distortion constants and allows us to establish that the (nuHD = 0, n = 1, J" = 0) level lies 13.7 cm(-1) above the (nuHD = 0, n = 0, J" = 0) level. Rovibrational energy level calculations performed using a recent ab initio potential energy surface confirm the picture emerging from the experimental data and provide good agreement with measured molecular parameters. The results emphasize the importance of quantum mechanical interconversion between two isomeric structures of a simple anion complex.     

Six-dimensional potential energy surface and rovibrational energies of the HCCN radical in the ground electronic state

We report large-scale quantum mechanical calculations for the HCCN radical in its ground electronic state. A six-dimensional potential energy surface based on MR-ACPF/cc-pVQZ ab initio energy points is developed and adjusted to reproduce experimental findings for and nu1 of HCCN. Rovibrational energy levels of HCCN and DCCN are computed for total rotational angular momentum J = 0-4 by making use of combined (functional + point wise) coordinate representations together with contraction schemes resulting from several diagonalization/truncation steps. The classical barrier to linearity is determined to be 287 cm(-1). Spectroscopic parameters are calculated for low lying states and compared with available experimental data. Energy patterns attributed to the nu4 bending mode and to the quasilinear nu5 bending mode are identified. It has been also found that nu2 and nu3 + (nu4(1),nu5(1))(0,0) are coupled in HCCN, while the mixing between nu3 and (2nu4(0), 2nu5(0))(0,0) is seen in DCCN.     

Internal rotation in NH4(+)-Rg dimers (Rg = He, Ne, Ar): potential energy surfaces and IR spectra of the nu 3 band

The intermolecular potential energy surfaces for the electronic ground states of the ammonium ion-rare gas dimers NH4(+)-He and NH4(+)-Ne are calculated at the MP2 and CCSD(T)/aug-cc-pVXZ (X = D/T/Q) levels of theory. The global minima of both potentials correspond to proton (vertex)-bound structures, Re = 3.13 A, De = 171 cm-1 (He) and Re = 3.21 A, De = 302 cm-1 (Ne). The face- and edge-bound structures are local minima and transition states for the internal rotation dynamics, corresponding to barriers of approximately 20 (He) and 50 cm-1 (Ne). The ab initio potentials are employed in numerical solutions to the rotation-intermolecular vibration Hamiltonian to determine the term values and the rotational and distortion constants for the lowest bound levels in the intramolecular ground vibrational state of both complexes. The results are used to assess the accuracy of two-dimensional (fixed-R) representations of the potentials for determining the internal rotor levels in the ground and nu 3 vibrational states. This model is employed to produce simulations of the IR nu 3 transitions, which are compared to the experimental spectra recorded using photofragmentation spectroscopy. In the case of NH4(+)-Ne the potential parameters are least-squares fitted to the experimental spectrum. The trends within the NH4(+)-Rg series (Rg = He, Ne, Ar) revealed by both the IR spectra and theoretical calculations are discussed.     

Infrared-vacuum ultraviolet-pulsed field ionization-photoelectron study of CH(3)I(+) using a high-resolution infrared laser

By using a high-resolution single mode infrared-optical parametric oscillator laser to prepare CH(3)I in single (J,K) rotational levels of the nu(1) (symmetric C-H stretching) =1 vibrational state, we have obtained rovibrationally resolved infrared-vacuum ultraviolet-pulsed field ionization-photoelectron (IR-VUV-PFI-PE) spectra of the CH(3)I(+)(X(2)E(32);nu(1)(+)=1;J(+),P(+)) band, where (J,K) and (J(+),P(+)) represent the respective rotational quantum numbers of CH(3)I and CH(3)I(+). The IR-VUV-PFI-PE spectra observed for K=0 and 1 are found to have nearly identical structures. The IR-VUV-PFI-PE spectra for (J,K)=(5,0) and (7, 0) are also consistent with the previous J-selected IR-VUV-PFI-PE measurements. The analysis of these spectra indicates that the photoionization cross section of CH(3)I depends strongly on DeltaJ(+)=J(+)-J: but not on J and K. This observation lends strong support for the major assumption adopted for the semiempirical simulation scheme, which has been used for the simulation of the origin bands observed in VUV-PFI-PE study of polyatomic molecules. Using the state-to-state photoionization cross sections determined in this IR-VUV study, we have obtained excellent simulation of the VUV-PFI-PE origin band of CH(3)I(+)(X (2)E(32)), yielding more precise IE(CH(3)I)=76 930.7+/-0.5 cm(-1) and nu(1) (+)=2937.8+/-0.2 cm(-1).     

High-resolution FTIR, microwave, and ab initio investigations of CH2 79BrF: ground, v(5) = 1, and v(6) = 1, 2 state constants

A spectroscopic study of CH279BrF in the infrared and microwave regions has been carried out. The rovibrational spectrum of the nu5 fundamental interacting with 2nu6 has been investigated by high-resolution FTIR spectroscopy. Owing to the weakness of the 2nu6 band, the v6 = 2 state constants have been derived from v6 = 1. For this reason, the rotational spectra of the ground and v6 = 1 states have been observed by means of microwave spectroscopy. Highly accurate ab initio computations have also been performed at the CCSD(T) level of theory in order to support the experimental investigation. As far as the nu5 band is concerned, the analysis of the rovibrational structure led to the identification of more than 3000 transitions, allowing the determination of a set of spectroscopic parameters up to sextic distortion terms and pointing out first-order c-type Coriolis interaction with the v6 = 2 state. With regard to the pure rotational spectra measurements, the assignment of several DeltaJ = 0, +1 transitions allowed the determination of the rotational, all the quartic, and most of the sextic centrifugal distortion constants, as well as the full bromine quadrupole coupling tensor for both the ground and v6 = 1 states.     

Photoinitiated predissociation of the NO dimer in the region of the second and third NO stretch overtones

Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.     

Nonrelativistic molecular quantum mechanics without approximations: electron affinities of LiH and LiD

We took the complete nonrelativistic Hamiltonians for the LiH and LiH- systems, as well as their deuterated isotopomers, we separated the kinetic energy of the center of mass motion from the Hamiltonians, and with the use of the variational method we optimized the ground-state nonadiabatic wave functions for the systems expanding them in terms of n-particle explicitly correlated Gaussian functions. With 3600 functions in the expansions we obtained the lowest ever ground-state energies of LiH, LiD, LiH-, and LiD- and these values were used to determine LiH and LiD electrons affinities (EAs) yielding 0.330 30 and 0.327 13 eV, respectively. The present are the first high-accuracy ab initio quantum mechanical calculations of the LiH and LiD EAs that do not assume the Born-Oppenheimer approximation. The obtained EAs fall within the uncertainty brackets of the experimental results.     

Observation and rovibrational analysis of the intermolecular NH3 libration band nu9(1) of H3N-HCN

The high-resolution far-infrared absorption spectrum of the gaseous molecular complex H(3)N-HCN is recorded by means of static gas-phase Fourier transform far-infrared spectroscopy at 247 K, using a synchrotron radiation source. The spectrum contains distinct rotational structures which are assigned to the intermolecular NH(3) libration band nu9(1) (nu(B)) of the pyramidal H(3)N-HCN complex. A rovibrational analysis based on a standard semirigid symmetric top molecule model yields the band origin of 260.03(10) cm(-1), together with values for the upper state rotational constant B' and the upper state quartic centrifugal distortion constants D'(J) and D'(JK). The values for the upper state spectroscopic constants indicate that the hydrogen bond in the H(3)N-HCN complex is destabilized by 5% and elongates by 0.010 A upon excitation of a quantum of libration of the hydrogen bond acceptor molecule.     

Rovibrational characterization of X 2Sigma+ 11BH+ by the extrapolation of photoselected high Rydberg series in 11BH

Optical-optical-optical triple-resonance spectroscopy of (11)BH isolates high Rydberg states that form series converging to rotational state specific ionization potentials in the vibrational levels of (11)BH(+) from nu(+)=0 through 4. Limits defined by a comprehensive fit of these series to state-detailed thresholds yield rovibrational constants describing the X (2)Sigma(+) state of (11)BH(+). The data provide a first determination of the vibrational-rotational interaction parameter alpha(e)=0.4821 cm(-1) and a more accurate estimate of omega(e)=2526.58 cm(-1) together with the higher-order anharmonic terms omega(e)x(e)=61.98 cm(-1) and omega(e)y(e)=-1.989 cm(-1). The deperturbation and global fit of series to state-detailed limits also yield a precise value of the adiabatic ionization potential of (11)BH of 79 120.3+/-0.1 cm(-1), or 9.810 33+/-1x10(-5) eV. High precision is afforded here by the use of graphical analysis techniques, narrow-bandwidth laser systems, and an analysis of newly observed, high principal quantum number Rydberg states that conform well with Hund's case (d) electron-core coupling limit.