Rg Atoms Research Articles

Infrared spectra of the phenol–Ar and phenol–N 2 cations: proton-bound versus π-bound structures

Infrared spectra of the phenol–Ar/N 2 cations (Ph +–Ar/N 2), produced in an electron impact ion source, are analyzed in the vicinity of the O–H stretch fundamental, ν 1. For Ph +–Ar two isomers are identified by their ν 1 frequency shifts upon complexation: the proton-bound global minimum (Δ ν 1=−65 cm −1) and the π-bound local minimum (Δ ν 1=+2 cm −1). The former isomer represents the first aromatic ion-rare gas (Rg) dimer where the Rg atom does not prefer binding to the aromatic π–electron system. The larger frequency shift of proton-bound Ph +–N 2 (Δ ν 1=−169 cm −1) compared to Ph +–Ar is consistent with a stronger intermolecular bond due to the additional charge–quadrupole interaction. Ab initio calculations support the interpretation of the experimental data for both species.

Dynamics of rare gas desorption from Pt(111) induced by collisions with photogenerated hot oxygen atoms

We present a classical trajectory study of the collision-induced desorption of rare gas (Rg=Ar, Xe) atoms from Pt(111) by ‘hot’ O atoms generated by photodissociating coadsorbed O 2. Empirical potential energy functions are used to describe the Rg/O/Pt(111) systems. The calculated angular and translational energy distributions of the desorbed Ar and Xe are in good agreement with experimental observations. It is found that the desorption dynamics involves two collisions. The first collision between O and Rg supplies the necessary kinetic energy for desorption, whereas the second collision between the departing Rg atom and a coadsorbed Rg atom results in a desorption angle of 25–35° from the surface normal for Ar and 35–45° for Xe. The light oxygen atom is found to chatter between the Xe atom and the surface during an O–Xe collision, enhancing the energy transfer.

Microsolvation of the methyl cation in neon: Infrared spectra and ab initio calculations of CH3+–Ne and CH3+–Ne2

Rotationally resolved infrared photodissociation spectra of the degenerate asymmetric C–H stretch vibration (ν3) of the CH3+–Ne and CH3+–Ne2 ionic complexes have been recorded. The rotational structure and vibrational frequencies are consistent with π-bound cluster geometries, where the Ne ligands are attached to either side of the 2pz orbital of the central C atom of the methyl cation, leading to C3v and D3h symmetric structures for the dimer and trimer. The intermolecular bonds in the ground vibrational state are characterized by averaged separations of Rc.m.=2.30 Å in the dimer and 2.34 Å in the trimer. The origins of the ν3 band are blueshifted by 11.5 and 21.5 cm−1 compared to the monomer frequency, indicating that vibrational excitation is accompanied by a small and additive destabilization of the intermolecular bond. Ab initio calculations at the MP2/aug-cc-pVTZ# level confirm that the π-bound configurations correspond to the global minimum structures for both the dimer (De=958.5 cm−1, Re=2.1347 Å, θe=91.4°) and the trimer (De=745.4 cm−1, Re=2.2322 Å, θe=90°). The calculated intermolecular potential energy surface of the dimer is characteristic for a disk-and-ball complex and reveals significant angular-radial coupling, which accounts for the large discrepancy between the vibrationally averaged and calculated equilibrium intermolecular separations, Rc.m.−Re≈0.17 Å. The comparison of the ionic CH3+–Rg dimers (Rg=He, Ne, Ar, Kr, Xe) with the isoelectronic CH3X molecules (X=H, F, Cl, Br, I) reveals that chemical bonding onsets with Rg=Ar and increases with the size of the Rg atom.

Heteronuclear rare-gas dimer bonding: Understanding the nature of the Rydberg states that dissociate to the highest energy level of the Xe*(5d) manifold

(1+1 ′ ) resonance enhanced multiphoton ionization (REMPI) spectra of jet-cooled KrXe and ArXe in the vicinity of the high energy Xe*5d[3/2]10←Xe(1S0) atomic line at 83889.99 cm−1 were obtained by exciting the neutral dimers with tunable coherent vacuum ultraviolet (VUV) radiation generated by four-wave sum mixing in mercury vapor, and then detecting the resultant ions in a time-of-flight (TOF) mass spectrometer. Precise excited state constants were derived from analyses of the resultant vibrational fine structure, while equilibrium bond lengths were estimated from Franck–Condon factor intensity simulations. Excited state symmetries were deduced from separate ultraviolet (UV) (2+1) REMPI spectra recorded with linearly and circularly polarized light. The results of this work confirm a recent model proposed by Lipson and Field, where the RgXe*(5d) states are predicted to be strongly destabilized relative to RgXe*(6p) due to strong 5d-6p Xe* l-mixing induced by the ground state Rg atom partner making up the dimer. Orbital mixing is also responsible for the observation of appreciably strong RgXe*(5d) spectra in both one- and two-photon excitation.

Interatomic potentials for the Na+—Rg complexes (Rg = He, Ne and Ar)

Interatomic potential energy curves are presented for the Na+—Rg (Rg = He, Ne and Ar) cationic complexes. The curves are calculated at the CCSD(T)/aug-cc-pVQZ level of theory, with correction for basis set superposition error being performed point-by-point. Ninety-six different bond lengths are used in the generation of the curves. From the curves rovibrational energy levels are calculated. These, in turn, are used to calculate the heat of formation of the cationic complexes, both by calculating partition functions under the assumption of a rigid rotor, harmonic oscillator, and also explicitly using the calculated rovibrational energy levels. The long range region of each of the curves is used to derive the D 4 and D 6 parameters, the former being used to derive the static polarizability a 1 of each of the Rg atoms and the latter the first dispersion coefficients, C 6(Na+—Rg).

Toward a global and causal understanding of the unusual Rydberg state potential energy curves of the heteronuclear rare gas dimers

Primitive calculations, based on simple physical concepts, have been found to explain the unusual excited state potential energy curves deduced from RgXe (Rg=Kr, Ar, Ne), two-photon spectra. The irregularities in the potentials and their dependence on the effective principle quantum number, n*, are attributed to repulsive exchange interactions between electrons in closed shell orbitals of the ground state Rg atom and the Xe* Rydberg electron. The internuclear distance dependence of these exchange interactions reflect the nodal structure of the radial probability distribution of the Rydberg orbital.

Geometries and Binding Energies of Rg·NO+ Cationic Complexes (Rg = He, Ne, Ar, Kr, and Xe)

The equilibrium geometries, harmonic vibrational frequencies, and interaction energies of the Rg·NO+ (Rg = He, Ne, Ar, Kr, and Xe) cationic complexes are calculated using a variety of all-electron basis sets and effective core potentials augmented by polarization functions. Calculations are performed at the MP2, QCISD, QCISD(T), CCSD, and CCSD(T) levels of theory for Rg = He and Ne using the all-electron aug-cc-pVXZ [X = D, T, Q, and 5 (for He only)] basis sets; and at the MP2 and QCISD(T) levels for Rg = Ar, Kr, and Xe, using mainly effective core potentials, augmented with polarized valence basis sets. For Ar the results are compared with previous all-electron calculations, to confirm that the basis sets used are adequate. The results indicate that all the complexes are of a skewed T-shaped structure, with the Rg atom on the nitrogen side of the molecule; the RgN−O bond angle increases with mass. The interaction energies range from 195 cm-1 for He·NO+ to 1980 cm-1 for Xe·NO+, in line with expectations based on the increasing polarizability of the Rg atom.

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.

Classical simulation of a cage effect in the dissociation of I2Rgn clusters (Rg = Ar,Kr,Xe; n⩽5)

The optical dissociation of I2 can be markedly suppressed, if the I2 molecule is weakly bound to one or more rare-gas (Rg) atoms (cage effect). A classical simulation of this process gives a fast disappearance of the cage effect and of the fluorescence intensity, as soon as the optical excitation energy Eω exceeds the dissociation energy of I2 by a certain amount of energy, which is controlled by the binding energy of I–Rgn in the ground state. For lower Eω the van der Waals potential between I2 and Rgn is strong enough in the asymptotic region to prevent the separating iodine atoms from dissociating. The oscillating I atoms can then transfer part of their vibrational energy to the Rg motion till the rare-gas atoms are evaporated. This mechanism gives a fluorescence spectrum of I2 with a cutoff at higher vibrational energies—smeared out only by the thermal excitation of the ground-state complex. The dependence of the spectrum on temperature and potential parameters has been investigated. At high excitation energies Eω a spectrum with an isolated peak can occur, if the van der Waals binding energy is increased or if more than one rare-gas atom is bound to I2. Other mechanisms which could result in a cage effect at higher Eω require a hard collision between an I and a rare-gas atom immediately after excitation. This is possible at high temperatures or for a linear conformation I–I–Rg. For an extended range of photon energies the simulations gave high yields of I–Rgn fragments from a I2–Rgn beam.

Hybrid quantum/classical studies of photodissociation and recombination of I2 (A) in rare gas matrices: A linear chain model

A hybrid quantum/classical model is developed for the photodissociation and recombination dynamics of an I2 molecule in low-temperature rare-gas (Rg) matrices. The simplified model consists of an I2 molecule embedded in a linear chain of Rg atoms. The aggregate is partitioned into a quantum system and a classical bath, which are self-consistently coupled. Two partitioning schemes are used. The first treats the I-I coordinate quantum mechanically and the Rg coordinates classically. The second and more reliable scheme includes in the quantum system both the I-I mode and the symmetric motion of the two nearest Rg atoms. Both models show substantial energy transfer from the dissociating I2 to the solvent, followed by coherent vibrational motion of the recombined I2. It is found that the one-dimensional quantum/classical scheme is consistent with its higher dimensional counterpart, although the latter shows much faster dephasing. © 1996 John Wiley & Sons, Inc.

Hybrid quantum/classical studies of photodissociation and recombination of I2 (A) in rare gas matrices: A linear chain model

A hybrid quantum/classical model is developed for the photodissociation and recombination dynamics of an I2 molecule in low-temperature rare-gas (Rg) matrices. The simplified model consists of an I2 molecule embedded in a linear chain of Rg atoms. The aggregate is partitioned into a quantum system and a classical bath, which are self-consistently coupled. Two partitioning schemes are used. The first treats the I-I coordinate quantum mechanically and the Rg coordinates classically. The second and more reliable scheme includes in the quantum system both the I-I mode and the symmetric motion of the two nearest Rg atoms. Both models show substantial energy transfer from the dissociating I2 to the solvent, followed by coherent vibrational motion of the recombined I2. It is found that the one-dimensional quantum/classical scheme is consistent with its higher dimensional counterpart, although the latter shows much faster dephasing. © 1996 John Wiley & Sons, Inc.

On the adequacy of pairwise additive potentials for rare gas–halogen systems: The effect of anisotropy of interactions between atoms

A simple model is presented for the potential energy functions of rare gas dihalides RgX2, which uses empirical potentials for diatomic fragments and takes properly into account anisotropic interactions between atoms, resulting in diabatic potentials which correlate with the ground state X2 molecule and Rg atom. Specific results are obtained for potential energy surfaces of ArX2 (X =F, Cl, Br, I) complexes and compared to those from several widely used models based on pairwise additive isotropic interactions. All these earlier models are found to underestimate the binding in the linear geometry, predicting a complete absence of a linear bound state; this feature is especially significant for ArF2 in which the anisotropic model predicts the linear configuration to be more stable. The new anisotropic model leads to Ar–X2 dissociation energies in good agreement with experiments.

Optical absorptions of Li atoms in mixed Ar/Xe matrices

We present the results of matrix isolation spectroscopy experiments on mixed Ar/Xe matrices containing Li atoms produced by laser ablation of solid lithium. Optical absorption spectra of Li/Ar/Xe matrices containing ∼0.01% guest Li atoms and ≊0, 3%, 10%, 30%, 50%, 70%, 90%, 97%, and 100% Xe as the matrix host are included. In all cases well defined ‘‘triplet’’ absorption features (i.e., three main peaks) are observed for the Li atom 2p←2s absorption. We also present new data on the photobleaching of the well known ‘‘red triplet’’ absorption in Li/Xe matrices, which show changes to the fine structure observed on the sharp 655 nm component. In these dilute guest systems, the Li atom absorption line shape is determined completely by guest–host interactions, which depend strongly on the local Li atom trapping site structure. In single Rg host matrices, it is possible that the trapping site structures may correspond to single or multiple Rg atom substitutional sites in otherwise crystalline regions of the rare gas solid. In these cases, the observed triplet line shapes would be due to the removal of the threefold degeneracy of the excited Li atom 2p state by dynamical distortions of the system away from the high symmetry equilibrium trapping site structures. In the mixed Ar/Xe matrices, the Li atom trapping sites necessarily have lower equilibrium or static symmetries due to the possibly amorphous nature of these solids, and to the differences in the Li–Ar and Li–Xe interactions. The observed spectra in mixed host matrices thus contain contributions from the many and varied Li atom trapping site structures, yet they still exhibit the familiar triplet absorption pattern. While we do not settle the long-standing question as to the crystalline vs amorphous nature of the single Rg host matrices, the present observations do provide new data for the comparison of the relative importance of static vs dynamic effects on the spectra of matrix isolated alkali atoms.

Potential energy curves of M(np 2P)⋅RG(2Π) excited states and M+⋅RG ground states (M=Li, Na; RG=He, Ne)

It has been established for some time that the bond energies for any given valence pπ excited state of Group 1 (2P) and Group 2 (1,3P) metal-atom/rare-gas (M⋅RG) van der Waals complexes tend to increase with the polarizability of the RG atom. It is also known that the binding energies of the corresponding M+⋅RG ground state ions are generally greater than those of the neutral M(pπ)⋅RG excited states with the same RG atom. However, there are two stark exceptions to these trends, both involving Group 1 metal atoms and the rare gas He; Li(2p 2P)⋅He(2Π) and Na(3p 2P)⋅He(2Π), which are the focus of the present study. We have conducted ab initio calculations of the potential energy curves of M(np 2P)⋅RG(2Π) and M+⋅RG states, where M=Li, Na and RG=He, Ne. We find that the unusual behavior of the pπ Group 1 metal atom states is due to (i) the lack of M(pπ)–RG(pπ) repulsive orbital overlap in the He case, and (ii) substantial additional attraction due to correlated motion of the RG atom’s electrons and the diffuse M(npπ) electron which is absent in the M+RG cases.

Spectroscopic characterization of the X 1Σ+ and C 1Π1 states of the ZnKr molecule

The X 1Σ+0 and C 1Π1 electronic states of the ZnKr van der Waals molecule have been characterized by laser-induced fluorescence spectroscopy. Spectroscopic constants, bond lengths, and dissociation energies are reported for both states. The substantial amount of data now available on M⋅RG ‘‘pure-π’’ excited states, where M=Zn, Cd, and Hg and RG=Ne, Ar, Kr, and Xe is also examined critically. It is proposed that the much larger dissociation energies and shorter bond lengths of the pure-π states compared to the ground states is due primarily to the fact that the RG atom, approaching in the nodal plane of the diffuse M(np) orbital, feels an effective positive charge when near the contracted M(ns) ‘‘core.’’ In the cases where re and De have both been determined, effective core charges are calculated to be 0.85–1.00 using a simple model potential with only a ‘‘core-ion/induced-dipole’’ attractive term. Fundamental vibrational frequencies calculated with the same potential also agree well with values determined experimentally for the pure-π states. Detailed comparisons of the properties of the pure-π states of Na⋅RG vs those of Zn⋅RG, Cd⋅RG, and Hg⋅RG molecules indicate that in the latter cases, the interaction also involves ‘‘back polarization’’ of the M(ns) core electron away from the approaching RG atom, which increases the apparent M(ns) charge.

Site effects in spectra of matrix-isolated diatomic molecules: A modelling approach

A model suitable for calculating electronic matrix shifts of diatomic molecules A-B in solid rare gases semi-quantitatively is presented. Guest-host interactions are modelled by summing over atom-pair potentials in a rare gas cluster of limited size embedded in a rigid crystal, and computing electrostatic interactions between the state-dependent dipole moment of the guest and the Rg atoms self-consistently. Numerical relaxation of the matrix yields state- and site-dependent solvation energies Δ V. The model is applied to NBr in solid argon, because (1) the dipole moments in the X 3Σ −, a 1Δ, and b 1Σ + states are available from theory, and (2) spectroscopic constants have been determined for several trapping sites in argon (see subsequent paper). The model also yields approximate vibronic matrix shifts. These informations can be utilized to assign trapping sites of diatomic molecules.