Raymond J. Bemish

The Ar–C2H2 intermolecular potential from high resolution spectroscopy and ab initio theory: A case for multicenter interactions

Infrared spectra have been obtained for the Ar–C2H2 complex, which include a combination band associated with the low frequency bending mode. These data are used, together with ab initio calculations and the results of previous studies of this system, to construct a two‐dimensional Hartree–Fock plus damped dispersion (HFD) intermolecular potential surface corresponding to the C–H stretch excited vibrational state. A high quality SCF surface, which includes ghost orbital corrections, has been used to fix the repulsive part of the potential. The remaining potential parameters were initially estimated with the aid of various combining rules and the collocation technique was used to solve the bound state problem for this potential and to calculate the spectrum of the Ar–C2H2 complex. To obtain good agreement between the calculated and experimental spectra it was necessary to distribute the dispersion interaction over the length of the acetylene subunit. The result is a double minimum potential upon which the ...

Photofragment vibrational, rotational, and translational distributions for N2–HF (v=1)

Two new methods have been developed for studying the vibrational predissociation dynamics of weakly bound complexes at the state‐to‐state level. The first is based upon the use of two F‐center lasers, one to photodissociate the complex by single quantum state excitation and the second to probe the final states of the resulting fragments. These pump–probe experiments are carried out with angular resolution of the photofragments, giving information on the associated vibrational, rotational and translational distributions. The second method makes use of a dc electric field to orient the parent molecules prior to photodissociation. In this way the two fragments recoil to different sides of the apparatus, allowing us to detect them independently. These experiments also provide us with valuable information on the nature of the photofragment angular distribution in the molecule fixed frame. The present application is to the study of the vibrational predissociation dynamics of N2–HF (v=1). A combination of the tw...

Photodissociation of molecules oriented by dc electric fields: Determining photofragment angular distributions

A quantum mechanical model has been derived for the photofragmentation of a molecule that is preoriented by a moderate to large dc electric field. The hybridization of the rotational wave functions in the dc electric field gives rise to net orientation and the resulting state mixing breaks the symmetry that is normally responsible for the fact that many laboratory measurements are only sensitive to a single anisotropy parameter (β). Consequently, the laboratory photofragment angular distributions resulting from molecules dissociated from ‘‘pendular’’ type states are sensitive to all of the terms in the Legendre expansion of the center‐of‐mass angular distribution, up to a value determined by the magnitude of the electric field. In essence, the electric field ‘‘locks’’ the molecule fixed frame onto the space fixed frame determined by the field direction. Additional advantages of this approach include the fact that the anisotropy is not quenched by rotation or hyperfine depolarization, owing to the fact tha...

Infrared spectroscopy and ab initio potential energy surface for Ne-C2H2 and Ne-C2HD complexes

The rotationally resolved spectra of Ne–C2H2 and Ne–C2HD were measured in the region of the asymmetric C–H stretch (ν3) band of the acetylene monomer. The transitions in the Ne–C2H2 spectrum are substantially broadened by vibrational predissociation, while those of Ne–C2HD are quite narrow. This difference is attributed to the fact that in the former dissociation proceeds through a “doorway” state, related to a Fermi resonance involving the bending vibrations of C2H2. In C2HD this Fermi resonance is absent. The potential energy surface (PES) for the Ne–acetylene complex has been computed using symmetry-adapted perturbation theory. This PES has been fit to an analytic form and applied in calculations of the rovibrational energy levels of Ne–C2H2 and Ne–C2HD. From these levels and calculated transition intensities we generated the near-infrared spectra of these complexes in the region of the ν3 band. These complexes may be considered as nearly free internal rotors. For Ne–C2H2 the results obtained from the ...

The N2H+–He intermolecular potential energy surface: A vibrational adiabatic correction

This paper presents a method for constructing computationally cheap adiabatically corrected ab initio potential energy surfaces (PES) for intermolecular vibrational states. The approach reasonably reproduces previously published experimental data for the N2H+–He complex in the ground and excited intramolecular vibrational states. A comparison made between a set of intermolecular PES’s with the N2H+ core frozen into the equilibrium geometry and a set where the N–H+ stretch is averaged demonstrates the importance of including this motion. This is also reflected in a considerable improvement in the agreement between the experimental and the calculated intermolecular bending and stretching frequencies and the origin red shift (νb,calc=117.9 cm−1, νs,calc=165.2 cm−1, Δνcalc=−93.0 cm−1). A comparison is also made between the Born–Oppenheimer angular radial separation (BOARS) angular average of the adiabatically corrected PES and the previously published rotational Rydberg–Klein–Rees (RRKR) PES. The results indi...

Ab initio and scaled potential energy surfaces for Ar–C2H2: Comparison with scattering and spectroscopic experiments

New coupled‐cluster ab initio potential energy surfaces (PES’s) were determined for the interaction of Ar with a rigid acetylene molecule. These PES’s were in addition modified by scaling the correlation energy. Based on both the original and scaled PES’s, close‐coupled calculations of the total differential scattering cross section were carried out. Rovibrational energy levels of the Ar–C2H2 complex were computed variationally. In addition, we simulated the ir spectra corresponding to excitation of the upper diad of the ν3/ν2+ν4+ν5 excited molecular vibrational states. The comparison of all these quantities with experiment shows generally good agreement for the several scaled PES’s. In addition, the sensitivity of the PES to the experimental data are investigated by varying the scaling factor. The original and scaled PES’s are also compared with several phenomenological PES’s and a previously published ab initio PES [F.‐M. Tao, S. Drucker, and W. Klemperer, J. Chem. Phys. 102, 7289 (1995)].

Collision-induced dissociation of positive and negative copper oxide cluster ions generated by direct laser desorption/ionization of copper oxide

Abstract Positive and negative cluster ions CunO+/−m (positive ions: n = 1–14, m = 0–8; negative ions: n = 1–2, m = 0–12) have been generated by direct laser desorption/ionization from a pellet of copper oxide and studied using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Energy-resolved collision-induced dissociation studies of these ions demonstrate interesting trends that may have implications for copper oxide cluster ion structures and stabilities.

Molecular control using dc electric fields: quenching of the tunneling in HF dimer

Abstract High resolution, optothermal infrared spectra have been obtained for the HF dimer in a uniform dc electric field, varying in strength from 0–150 kV/cm. These fields not only serve to orient the molecules in the space fixed frame, but also to quench the tunneling motion that is normally characteristic of the HF dimer. This is attributed to the fact that the electric field makes the double well tunneling problem asymmetric in the limit that the molecules are well oriented. A simple 1-D model is used to qualitatively explain the results.

The ethylene–carbon dioxide complex: A double internal rotor

A rotationally resolved near‐infrared spectrum has been obtained for the ethylene–CO2 van der Waals complex. The structure obtained for this complex has the carbon dioxide lying above the plane of the ethylene molecule, with the CO2 axis parallel to this plane. The individual subbands observed in the spectrum are shifted from the expected rigid rotor positions, indicative of internal rotation of the two monomer units within the complex. A double rotor model is presented that accounts for the shifts and provides an estimate of the barrier to internal rotation. The results indicate that the complex is completely delocalized along the internal rotational coordinate. Ab initio calculations are also presented that are in good agreement with this picture.

High resolution infrared molecular beam spectroscopy of cyanoacetylene clusters

High resolution infrared optothermal spectroscopy has been used to study small clusters of cyanoacetylene, (HCCCN)n, containing three or more monomer units. For the linear HCCCN trimer the fundamental of the free C–H stretch vibration has been rotationally resolved and analyzed, yielding a ground vibrational state rotational constant (B0) of 94.031(13) MHz. In addition, an inner C–H stretch fundamental band of the linear trimer (rotationally unresolved) has been identified by observing the effect of large electric fields on the band shape. This assignment is also consistent with spectral intensity measurements as a function of molecular beam stagnation pressure. Predissociation lifetimes of the upper states of these two bands were determined from the observed homogeneous linewidths. Several other rotationally unresolved vibrational bands have also been observed. The stagnation pressure and electric field dependence of these spectra are used to estimate the cluster sizes and to make tentative structural as...