Van Der Waals Molecules Research Articles

Spectroscopic characterization of the excited valence MgKr(3dδ 3Δ1) and MgKr(3dπ 3Π) van der Waals states

The Mg(3s3dπ 3DJ)⋅Kr(3Π0+,0−) and Mg(3s3dδ 3DJ)⋅Kr(3Δ1) valence excited states have been characterized via resonance enhanced two-photon ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3pπ 3PJ)⋅Kr(3Π0+,0−) metastable states of the MgKr van der Waals molecule. Because the excited Mg(3d) orbital is quite diffuse and the Kr atom can approach along the nodal axis of the aligned 3d orbital, minimizing repulsion, the MgKr(3s3dδ 3Δ1) state is very strongly bound (D0=1874±80 cm−1), closely approaching the bond energy of the MgKr+ core ion (for which D0=1891±80 cm−1). In contrast, the MgKr(3s3dπ 3Π0−) state is more weakly bound (D0=587±80 cm−1), although it has a greater bond strength than the MgKr(3s3pπ 3Π0−) lower state (for which D0 is 250±80 cm−1). We have also observed interesting spin–orbit effects which are attributed to mixing of Kr character into molecular orbitals which are nominally of Mg* excited-state character.

Spectroscopic characterization of the singly excited CaAr(4dπ 3Π), CaAr(4dδ 3Δ1) states and the doubly excited CaAr(4pπ4pπ 3Σ−) state

The singly excited Ca(4s4dπ 3DJ)⋅Ar(3Π0−), and Ca(4s4dδ 3DJ)⋅Ar(3Δ1) valence states and the unusual doubly excited Ca(4pπ4pπ 3PJ)⋅Ar(3Σ−) valence state have been characterized via Resonance Enhanced Two-Photon Ionization (R2PI) spectroscopy of transitions from the long-lived Ca(4s4pπ 3PJ)⋅Ar(3Π0−) metastable state of the CaAr van der Waals molecule. Because the Ar atom can approach along the nodal axis of the quite diffuse Ca(4dδ) orbital with little repulsion, the CaAr(4s4dδ 3Δ1) state is fairly strongly bound (D0=609±50 cm−1), but less bound than the CaAr+ “core” ion (D0=775±50 cm−1). The CaAr(4s4dπ 3Π0−) state is less bound (D0=450±50 cm−1), but has a bond strength much greater than that of the lower-energy CaAr(4s4pπ 3Π0−) state (D0=60±50 cm−1). The doubly excited CaAr(4pπ4pπ 3Σ−) state is extremely strongly bound (D0=1277±50 cm−1), and this is attributed to the lack of a valence Ca(4s) electron. The Ar atom can therefore approach with minimal repulsion along the nodal axes of the two transversely aligned Ca(4pπ) electrons. The Ca(4pπ4pπ 3Σ−) state is also strongly predissociated, with a lifetime less than 10 ps.

Spectroscopic characterization of the metastable 3pπ 3Π+,0− valence states and the 4s3Σ+ Rydberg states of the MgKr and MgXe van der Waals molecules

The first metastable valence excited states and the first Rydberg states of the MgKr and MgXe molecules have been characterized by resonance two-photon photoionization (R2PI) spectroscopy. The Mg(3s3p 3PJ)⋅RG(3Π0+,0−) metastable states, produced by expanding the products of a laser-ablated magnesium rod in Kr/Ar or Xe/Ar gas mixtures into a supersonic expansion, were excited by a dye laser pulse to several vibrational levels of the Mg(3s4s 3S1)⋅RG(3Σ+) Rydberg states, with detection by ionization with a second dye laser pulse. Spectroscopic constants, bond energies, and bond lengths are reported for both states of MgKr and MgXe. The Σ+3 Rydberg states are much more strongly bound than the lower Π0−3 valence states, and in fact are essentially as strongly bound as the ground states of the analogous MgRG+ ions, characterized previously in the same apparatus. This clearly indicates that the RG atoms can readily penetrate the diffuse Mg(4s) Rydberg electron cloud. The interesting and unusual spin–orbit and “spin–spin” effects observed are attributed to mixing of some RG character into wave functions of predominantly Mg* excited state character. Bonding and spin–orbit interactions in the MgAr, MgKr, and MgXe first triplet metastable and Rydberg states are discussed.

Three-dimensional quantum dynamics study of vibrational predissociation of HeI2 van der Waals molecule for low vibrational excitation using the time-dependent wave packet method

Three-dimensional quantum mechanical calculations for vibrational predissociation of He12(B) van der Waals molecules are presented using the time-dependent wave packet technique within the golden rule approximation. The total and partial decay widths, lifetimes, rates and their dependence on initial vibrational states were obtained for HeI2 at low initial vibrational excited levels. Our calculations show that the calculated total decay widths, lifetimes and rates agree well with those extrapolated from experimental data available. The predicted total decay widths as a function of initial vibrational states exhibit highly nonlinear behavior. The very short propagation time (less than 1 ps) required in the golden rule wave packet calculation is determined by the duration time of the final state interaction between the fragments on the vibrationally deexcited adiabatic potential surface. The final state interaction between the fragments is shown to play an important role in determining the final rotational distribution. This interpretation clearly explains the dynamical effect that the final rotational distribution shifts to the lower rotational energy levels as the initial vibrational quantum numberu increases.

Nuclear Quadrupole Hyperfine Structure in the Microwave Spectrum of Ar–N2O

Nuclear quadrupole hyperfine structure in the rotational spectrum of Ar–N2O has been resolved in the 7–18 GHz region using a Fourier transform microwave spectrometer. Analysis of the hyperfine lines enables not only the determination of the rotational and centrifugal distortion constants to greater precision than previous studies, but also provides values for the nuclear quadrupole coupling constants of each14N nucleus. The coupling constants for Ar–N2O, when compared to those for free N2O, show that the electric field gradient at the terminal nitrogen nucleus in the N2O subunit is not affected by the presence of Ar, whereas that at the central nitrogen nucleus is affected slightly. This result, in contrast to recent findings in HCCH–N2O, supports the usual practice of extracting geometrical information for a van der Waals molecule from the nuclear quadrupole coupling tensor(s) of the complex.

Van der Waals molecules and collision pairs of noble-gas atoms ahead of a shock wave front

The effect of creation of an excess concentration of free electrons in an anomalously thick layer (≈5 cm) ahead of an explosively driven shock wave in noble gases is discussed and interpreted. This effect is the ionization of excited 1u-state molecules produced due to the absorption of a small intensity flux (as compared to the ionization one) of photons (with energies substantially lower than the atom ionization threshold) by unexcited colliding complexes and van der Waals molecules. A model is proposed which explains the excitation of xenon molecules ahead of the radiationless shock wave of an open discharge. The absorption spectra of colliding complexes and van der Waals molecules adjacent to each other near the atomic absorption lines can be resolved into two spectra, and these spectra can be changed by an increase in gas temperature. As a result, radiation capable of exciting van der Waals molecules penetrates through the shock wave of an open discharge and excites xenon molecules there. The present work develops further the knowledge concerning the radiation energy transport in the shock wave front. It also proves that in front of an explosively driven shock wave a great number of excited molecules of noble gases are actually formed, and this means considerable progress toward a VUV laser with optical pumping.

New and Unusual Bonding in Open Shell van der Waals Molecules Revealed by the Heavy Atom Effect: The Case of BAr

Calculations are reported for the excited 14Π and C2Δ electronic states of BAr and the (nominally 2s2p2) 4P and 2D states of atomic boron. It was found necessary to extend an augmented valence triple-zeta orbital basis set to correctly describe the C2Δ state. This enhancement is necessitated by the near degeneracy of the valence 2s2p2 2D and Rydberg 2s23d 2D electron configurations in atomic boron, both of which are required to describe BAr C2Δ. Unexpectedly strong bonding in the C2Δ state is found. This bonding is shown to have its root in a type of coordinate covalency, dative bonding, that is sensitive to electron correlation effects and is reflected in a large external heavy atom effect in the spin−orbit coupling of the 14Π and C2Δ states.

Infrared and Microwave Spectra of the N2–Propyne van der Waals Cluster

The infrared spectrum of14N2–CH3CCH in the 3.0-μm region has been observed and the rovibrational transitions have been assigned for theA(m= 0) andE(m= ±1) methyl top internal rotor states of this near slipped parallel van der Waals molecule. TheE-state transitions were used to estimate the barrier to internal rotation as 10 cm−1. The observedA-state microwave transitions of14N2–CH3CCH,15N2–CH3CCH, and15N2–CH3CCD are split into doublets. This doublet splitting, which is too small to be observed in the infrared spectrum, is attributed to the internal motion of the N2moiety about an axis connecting the centers-of-mass of the molecules in the cluster; i.e., the N2molecule rotates out of the heavy atom plane of the cluster. The splittings in the microwave spectra were fit to a barrier height of 71 cm−1for the hindered N2motion.

Spectroscopic characterization of the Δ3(4d), Π3(4d), Σ+3(4d), and Π3(5p) Rydberg states of the MgAr van der Waals molecule

The Mg(3s4dσ 3DJ)⋅Ar(3Σ+), Mg(3s4dπ 3DJ)⋅Ar(3Π), Mg(3s4dδ 3DJ)⋅Ar(3Δ), and Mg(3s5pπ 3PJ)⋅Ar(3Π) Rydberg states have been characterized via Resonance Enhanced Two-Photon Ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3pπ 3PJ)⋅Ar(3Π0+,0−) metastable states of the MgAr van der Waals molecule. The 4dπ, 4dδ, and 5pπ states are all strongly bound (D0=1230±50, 1200±40, 1270±50 cm−1, respectively). These bond energies are very similar to that of the ground-state MgAr+ “core’’ ion (D0=1240±40 cm−1), indicating very effective penetration of transversely aligned, diffuse Rydberg electron clouds on the Mg atom by the Ar atom, even for the low-lying n=4,5 Rydberg states. The Mg(3s4dσ 3DJ) ⋅Ar(3Σ+) state was substantially less bound, D0=800±40 cm−1, showing there is still some residual Mg(4dσ)/Ar(3pσ)2 electron–electron repulsion preventing penetration of the axially aligned Mg(4dσ) Rydberg electron cloud by the Ar atom. Successful computer simulations of the rotational structure of several of the vibrational transitions to the 4dΔ and 4dΣ states, assuming Hund’s case “b’’ upper-state character, resulted in R0′ values of 2.80±0.04 Å and 2.90±0.05 Å, respectively (compared to R0=2.82±0.01 Å determined previously by others for the MgAr+ ion). Because the 4dπ and 5pπ states have similarly large bond strengths as well as similar asymptotic Mg(4d 3DJ) and Mg(5d 3PJ) atomic energies, they have “mixed’’ 4dπ/5pπ character.

Dimers of oxygen molecules in the pores of Ca6A zeolite

T. Takaishi, J. Chem. Soc., Faraday Trans., 1997, 93, 1257 DOI: 10.1039/A605344K

Research Activity Using Molecular Beam and Laser Techniques: A Progress Report

We shall briefly describe the main research areas, now in progress in our molecular beam and laser group. The current activity includes our traditional molecular beam work as well as other experiments carried out in the new labs of our Institute. Essentially we present here the current progress on Crossed-beam and Beam-gas Chemilumine scence, Laser-induced Beam Surface Reaction, Laser Analytical Chemistry, Laser-initiated Reactions Studies by Chemiluminescence and Laser-induced Fluorescence, Spectroscopy and Structure of van der Waals molecules. For each section recent results are presented. Due to the basic scope of the present progress report the emphasis is made on the new experimental developments as well as on the capabilities of both basic and applied research techniques now operative in our group.

Spectroscopic characterization of the excited Mg(3s3d 3D J)⋅Ar(3Π), Mg(3s3d 3D J)⋅Ar(3Δ), and Mg(3s4p 3P J)⋅Ar(3Π) van der Waals states

The Mg(3s3dπ 3DJ)⋅Ar(3Π), Mg(3s3dδ 3DJ)⋅Ar(3Δ), and Mg(3s4pπ 3PJ)⋅Ar(3Π) excited states have been characterized via resonance enhanced two-photon ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3pπ 3PJ)⋅Ar(3Π0+,0−) metastable states of the MgAr van der Waals molecule. Because the outer excited Mg(3d) and Mg(4p) orbitals are (similarly) quite diffuse and the Ar atom can approach along the nodal axes of each of the aligned orbitals, minimizing repulsion, the Mg(3s3dδ 3DJ)⋅Ar(3Δ) and Mg(3s4pπ 3PJ)⋅Ar(3Π) states are both very strongly bound (D0=1140±40, 1250±60 cm−1, respectively), approaching the bond energy of the MgAr+ ‘‘core’’ ion (for which D0=1240±40 cm−1). In contrast, the Mg(3s3dπ 3DJ)⋅Ar state is more weakly bound (D0=290±40 cm−1), although it has a greater bond strength than the Mg(3s3pπ 3PJ)⋅Ar(3Π0−) lower state (for which D0 is 160±40 cm−1). The effective spin–orbit constant of the Mg(3s3dπ 3DJ)⋅Ar(3Π) state is much larger than expected from the Mg(3s3d 3DJ) multiplet splittings, and also increases in magnitude as v′ decreases; possible reasons for this are discussed. It is suggested that the predissociation of the Mg(3s3dπ 3DJ)⋅Ar(3Π0+) state [the Mg(3s3dπ 3DJ)⋅Ar(3Π0−) state does not predissociate] is caused by a curve-crossing with the repulsive Mg(3s3dσ 1D2)⋅Ar(1Σ0++) state, which lies at a lower asymptotic energy because of substantial mixing of Mg(3p3p 1D2) character into its wave function.

One-atom cage effect in collinear I2(B)–Ar complexes: A time-dependent wave packet study

Two-dimensional time-dependent wave packet calculations are carried out on a collinear model of the I2(B)–Ar complex to investigate the possible kinematic origin of the one-atom cage effect in small van der Waals molecules. Three different excitation wavelengths are considered (496.5, 488, and 476.5 nm), and the dynamics are assumed to be restricted to the I2 B state electronic surface, with no nonadiabatic transitions following the pump excitation. Good agreement with experiment is obtained. To investigate the sensitivity of observable final state distributions on the weak intermolecular potential between I2 and Ar, three slightly different B state I–Ar interactions are employed for the case of 488 nm excitation. It is found that relatively small changes in the form and magnitude of the weak van der Waals interactions can have a large effect on the final state distributions. These results suggest that the experimental data on I2–Ar photodissociation–recombination can be explained by a purely kinematic one-atom cage effect on the B state electronic surface for a collinear population of I2–Ar clusters, without the need to introduce nonadiabatic electronic effects.

Estimate of P-and P, T-odd effects in diatomic van der Waals molecules

The magnitudes of the possible P-and P, T-odd effects in diatomic van der Waals molecules are estimated. The estimates fall between the values corresponding to atoms and molecules/radicals. Arguments are presented showing that the loss of approximately two orders of magnitude in the strength of the effect for van der Waals molecules with an unpaired electron, such as CsXe, as compared with molecules/radicals can be partially compensated by a number of experimental advantages of working with van der Waals molecules.

Potential energy curve of the ground state of HgAr determined from fluorescence spectra

Abstract A 0 + ( 3 Π) → X 0 + ( 1 Σ + ) and B 1( 3 Σ + ) → X 0 + ( 1 Σ + ) fluorescence spectra of HgAr van der Waals molecules were previously produced in a pulsed supersonic molecular beam crossed with a pulsed dye-laser beam, following excitation of single vibronic levels. The dispersed fluorescence displayed characteristic Condon internal diffraction (CID) patterns consisting of bound-free reflection type, continuous spectra, and also bound-bound discrete features. An analysis of the AO+ → X0+ and B1 → XO+ bound-bound spectra indicates that a Morese function is an adequate representatation of the X0+ potential energy (PE) curve below the dissociation limit. In simulation of the AO+ → X0+ bound-free spectra of the Morse, Lennard-Jones (n − 6) and Maitland-Smith functions were tested, and the Maitland-Smith potential was found to be a good representation of the repulsive wall of the X0+ PE curve above the dissociation limit over the internuclear separation range R = 2.8–3.5 A .

Doubly excited valence states of neutral van der Waals molecules: Mg(3pπ,3pπ 3P J)⋅Ar(3Σ−)

The first of a new class of doubly excited valence states of van der Waals molecules, Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−), has been discovered and characterized by resonance two-photon ionization spectroscopy. This state has been found to be quite strongly bound (D0=2850±100 cm−1 and to have a very short bond length (R0=2.41±0.02 Å). The analogous singly excited state, Mg(3s3pπ 3PJ)⋅Ar(3Π0−), in stark contrast, has a very much smaller bond strength (D0=160±40 cm−1) and a longer bond length (R0=3.66±0.02 Å). In fact, the new Mg(3pπ3pπ 3PJ)⋅Ar state is more than twice as bound than even the Mg(3s 2S1/2)⋅Ar+ ground-state ion (D0=1240±40 cm−1, R0=2.82 Å). The strong bonding is due both to the transverse alignment of the Mg(3pπ) electrons and particularly to the lack of a Mg(3s) electron. The resulting loss of Mg(3sσ)/Ar(3pσ)2 repulsive forces facilitates close approach of the Ar atom. This is turn allows attractive Mg(3pπ)2/Ar dispersive forces and Mg2+/Ar ion/induced-dipole type forces to continue to much smaller internuclear distances, creating a strong, short van der Waals bond. Possible mechanisms for the parity-dependent predissociation and the apparent ‘‘spin–spin’’ type splittings observed for the Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−) state are also discussed.

Strong bonding in a doubly excited valence state of a van der Waals molecule

The results of ab initio calculations, using extended Gaussian basis sets and including correlation energy through a variety of means, provide strong support for the Mg(3 pπ,3 pπ 3 P J) · Ar( 3Σ −) electronic-state assignment made by Massick and Breckenridge to spectral features in the resonant two-photon photoionization spectrum of vibrationally cold Mg(3 s,3 pπ 3 P J) · Ar( 3Π) van der Waals complexes. The doubly excited Mg(3 pπ,3 pπ 3 P J) · Ar( 3Σ −) state is found to be much more strongly bound than the singly excited Mg(3 s,3 pπ 3 P J) · Ar( 3Π) state, which, in turn, is much more bound than the Mg(3 s 2 1 S O ) · Ar( 3Σ +) ground state.

A new class of strongly bound, doubly excited valence states of neutral van der Waals molecules: Mg(3pπ,3pπ 3P J)⋅Ar(3Σ)

The first of a new class of strongly bound, double excited valence states of neutral van der Waals molecules, Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−), has been characterized by R2PI (resonance two-photon ionization) spectroscopy via excitation of metastable Mg(3s3pπ 3PJ)⋅Ar(3Π0+,0−) states in a supersonic jet. The MgAr(3Σ−) state has a bond energy of ∼2900 cm−1, more than twice that of the MgAr+ ion.

High resolution spectroscopy of the He79Br2 van der Waals molecule: An experimental and theoretical study

The structure, dissociation dynamics, and intermolecular potential energy surfaces of the He79Br2 van der Waals molecule have been studied using high resolution, two color, pump–probe laser induced fluorescence spectroscopy and three dimensional quantum mechanical calculations. A conical nozzle produces higher centerline cluster densities than a standard nozzle, and allows data collection further downstream from the nozzle. This yields improved signal to noise ratios and lower Doppler widths. He79Br2 is found to have a T-shaped average geometry with He to Br2 center-of-mass distances of 3.98 Å and 4.11 Å for the X and B states, respectively, somewhat longer than previously reported. Spectra were also obtained for excitation to excited bending levels of the van der Waals coordinate. However, these spectra have yet to be rotationally assigned. Vibrational predissociation line widths for the B state of He79Br2 have been measured for three new vibrational levels and range from 0.036 cm−1 for B, v′=8 to 0.062 cm−1 for B, v′=12. These values are somewhat larger than was expected based on previous HeBr2 linewidth measurements for higher vibrational levels. Forms for the potential energy surface that have previously been used to simulate the spectra of HeCl2 have been applied to the HeBr2 data. For the HeBr2 X state, two potentials are tested. First, a slightly anisotropic, one center Morse–Spline–van der Waals potential with angle dependent parameters is used. Second a much more anisotropic potential obtained from ab initio calculations is tested. The more anisotropic potential produces a significantly better fit to the data. The B state potential is constructed using Morse atom–atom potentials for the short range part of the He–Br interaction. This simple potential is sufficient to fit the main excitation band, but does not yield a fit to spectra involving vibrationally excited van der Waals modes.

A general method for constructing multidimensional molecular potential energy surfaces from ab initio calculations

A general interpolation method for constructing smooth molecular potential energy surfaces (PES’s) from ab initio data are proposed within the framework of the reproducing kernel Hilbert space and the inverse problem theory. The general expression for an a posteriori error bound of the constructed PES is derived. It is shown that the method yields globally smooth potential energy surfaces that are continuous and possess derivatives up to second order or higher. Moreover, the method is amenable to correct symmetry properties and asymptotic behavior of the molecular system. Finally, the method is generic and can be easily extended from low dimensional problems involving two and three atoms to high dimensional problems involving four or more atoms. Basic properties of the method are illustrated by the construction of a one-dimensional potential energy curve of the He–He van der Waals dimer using the exact quantum Monte Carlo calculations of Anderson et al. [J. Chem. Phys. 99, 345 (1993)], a two-dimensional potential energy surface of the HeCO van der Waals molecule using recent ab initio calculations by Tao et al. [J. Chem. Phys. 101, 8680 (1994)], and a three-dimensional potential energy surface of the H+3 molecular ion using highly accurate ab initio calculations of Röhse et al. [J. Chem. Phys. 101, 2231 (1994)]. In the first two cases the constructed potentials clearly exhibit the correct asymptotic forms, while in the last case the constructed potential energy surface is in excellent agreement with that constructed by Röhse et al. using a low order polynomial fitting procedure.