The state-to-state predissociation dynamics of OC–HF upon HF stretch excitation

A full-dimensional ab initio intermolecular potential energy surface and rovibrational spectra for OC-HF and OC-DF

We present a full-dimensional ab initio intermolecular potential energy surface (IPES) for the OC-HF van der Waals complex. 3167 ab initio points were computed at the frozen-core (FC) explicitly correlated coupled cluster [FC-CCSD(T)-F12b] level, with the augmented correlation-consistent polarized valence quadruple-zeta basis set plus bond functions. Basis set superposition error correction was also considered by the full counterpoise procedure. Gaussian process regression (GPR) was used to map out the potential energy surface, while a multipole expansion method was employed to smooth the ab initio noise of intermolecular potential in the long range. The global minimum of -1248.364 cm^-1 was located at the linear configuration with the C atom pointing toward the H atom of the HF molecule. In addition, a local minimum of -602.026 cm^-1 was found at another linear configuration with the O atom pointing toward the H atom of the HF molecule. The eigenstates were calculated on the vibrational averaged four-dimensional IPESs with the mixed radial discrete variable representation/angular finite basis representation method and Lanczos propagation algorithm. The dissociation energy D₀ was calculated to be 701.827 cm^-1, well reproducing the experimental value of 732 ± 2 cm^-1. The dipole moment surfaces were also fitted by GPR from 3132 ab initio points calculated using the coupled cluster method [CCSD(T)] with AVTZ basis set plus bond functions. The frequencies and relative line intensities of rovibrational transitions in the HF (DF) and CO stretching bands were further calculated and compared well with the experimental results. These results indicate the high fidelity of the new IPES.

Rovibrational spectra of the N2–HF complex at the vHF=3 level

We report the analyses of the three intermolecular combination bands of the hydrogen-bonded N2-HF complex at vHF=3, observed by molecular beam intracavity laser induced fluorescence. The origin of the HF intermolecular bending combination band, (3001(1)0)<--(00000), is 11 548.45(3) cm(-1), 328.2 cm(-1) higher than that of the (30000)<--(00000) transition with an origin at 11 220.250(1) cm(-1). The average rotational constant of the (3001(1)0) level is 0.103 63(1) cm(-1), a 4.8% reduction from B(30000)=0.109 21(1) cm(-1). Perturbations are observed as line splittings, increased line widths, and reduced peak intensities of a number of lines of the e and f components of (3001(1)0). In addition, the centrifugal distortion coefficients of both components are large, negative, and different. The N2 intermolecular bend transition (30001(1))<--(00000) has an origin at 11 288.706(1) cm(-1), 68.456(2) cm(-1) above that of the (30000)<--(00000) transition. This is the lowest combination state at v(HF)=3 level. It is unperturbed, yielding B(30001(1))=0.110.10(1) cm(-1). The transition to the intermolecular stretching state, (30100)<--(00000), has an origin at 11 318.858(1) cm(-1) with B(30100)=0.105 84(1) cm(-1). Both the (30100) and (30000) levels show an isolated perturbation at J=4. The Lorentzian component of the line widths, which show considerable variation with soft mode, are GammaL(30000)=490(30) MHz, GammaL(30100)=630(30) MHz, GammaL(3001(1)0)=250(30) MHz, and GammaL(30001(1))=500(50) MHz.

Asymmetry in angular rigidity of hydrogen-bonded complexes

The asymmetry in angular rigidity of the proton donor and proton acceptor of hydrogen-bonded hydrogen fluoride binary complexes is investigated. The intermolecular bending frequency of HF, as the proton donor, is linearly proportional to the square root of the dissociation energy, whereas that of the proton acceptor is always much lower. The asymmetry, measured by the ratio of bending elastic constants of HF to that of the proton acceptor, is generally >2, and varies pronouncedly with the acceptors reaching values >20. Molecules with nitrogen as the bridged acceptor atom show an angular rigidity nearly one order of magnitude greater than the group with oxygen as the proton acceptor.

Overtone spectroscopy of H2O clusters in the vOH=2 manifold: Infrared-ultraviolet vibrationally mediated dissociation studies

Spectroscopy and predissociation dynamics of (H2O)2 and Ar-H2O are investigated with vibrationally mediated dissociation (VMD) techniques, wherein upsilon(OH) = 2 overtones of the complexes are selectively prepared with direct infrared pumping, followed by 193 nm photolysis of the excited H2O molecules. As a function of relative laser timing, the photolysis breaks H2O into OH and H fragments either (i) directly inside the complex or (ii) after the complex undergoes vibrational predissociation, with the nascent quantum state distribution of the OH photofragment probed via laser-induced fluorescence. This capability provides the first rotationally resolved spectroscopic analysis of (H2O)2 in the first overtone region and vibrational predissociation dynamics of water dimer and Ar-water clusters. The sensitivity of the VMD approach permits several upsilon(OH) = 2 overtone bands to be observed, the spectroscopic assignment of which is discussed in the context of recent anharmonic theoretical calculations.

Spectroscopy of the OC–HF hydrogen-bonded complex at vHF=3

The v(HF)=3 levels of the linear OC-HF complex are observed in the range of 10,800-11,500 cm(-1) using intracavity Ti-sapphire laser-induced fluorescence. The vibrational predissociation linewidths of both (30000) and (3001(1)0) states exceed 5 GHz; thus, the measured spectra are not rotationally resolvable. Under the assumption that these levels are not strongly perturbed, the rotational constants of the two levels are determined to be 0.1100(1) cm(-1) for (30000), 0.1081(1), and 0.1065(1) cm(-1) for f and e sublevels of (3001(1)0), respectively, through band contour fitting. The (30000)<--(00000) band origin is at 10,894.46(1) cm(-1), showing a HF wave number redshift of 478.3 cm(-1). The 4.07 redshift ratio of v(HF)=3 to that of v(HF)=1 indicates a significantly nonlinear increase of the intermolecular interaction energy through HF valence excitation. An ab initio interaction potential surface for HF valence coordinates varying from 0.8 to 1.25 A is used to examine vibrational dynamics. The HF valence vibration v(1) is treated perturbatively, showing that the vibrational redshifts are determined essentially in first order with only a very small second-order contribution. The (3001(1)0)<--(00000) combination transition is observed with the band origin at 11,432.66(1) cm(-1), giving the HF intermolecular bending mode to be 538.2 cm(-1). The high frequency of this vibration, compared to that in similar HF complexes, shows the strong angular anisotropy of the intermolecular interaction potential of OC-HF with respect to the HF subunit. The lifetime of the (3001(1)0) level increases to 28 ps from 14 ps for (30000).

Photofragment translational spectroscopy of weakly bound complexes: probing the interfragment correlated final state distributions

The vibrational predissociation dynamics of weakly bound complexes is well known to be highly nonstatistical. In particular, the associated photofragment final state distributions are often far from statistical, consequently reflecting the nature of the dissociation process. For binary complexes consisting of two molecules, a complete description of the final state of the system must include the associated interfragment correlations, specifically between their internal states. Information of this type is imprinted in the translational energies of the fragments, which can be measured using a number of recently developed translational spectroscopy methods. These data can provide detailed insights into the nature of the bond rupture process, as well as accurate values for the dissociation energy of the complexes. The focus of the present review is on experiments that provide correlated final state distributions for weakly bound binary complexes. Where possible, comparisons with theoretical calculations are made.