J. Bulthuis

Steric asymmetry in rotationally inelastic state-resolved NO-Ar collisions

Abstract The first direct experimental evidence is reported of large orientation (head-tail) effects in rotationally inelastic collisions, for the specific case of NO and Ar. NO is selected in the 2 Π 1 2 j = 1 2 − state and oriented in an electric field. The steric effect, measured for collisions to three distinct final rotational states, appears to depend on the j ′ value and the parity of the final state. This behaviour also follows from quantum-mechanical scattering calculations, but the calculated values do not coincide with the corresponding experimental ones.

ELECTRIC FIELD DEPENDENCE OF REACTIVITY OF STATE-SELECTED AND ORIENTED METHYLHALIDES

The energies and eigenfunctions of hyperfine levels belonging to the J=‖K‖=1 rotational manifold of the methylhalides have been calculated as a function of the strength of an applied electric (dc) field. From the eigenfunctions, the molecular orientational distribution function P(cos ϑ), where ϑ is the angle of the principal axis relative to the electric field, has been obtained. In the case of interest (J=1), the Legendre expansion of P(ρ), with ρ≡cos ϑ, can be characterized by two moments only: 〈P1〉 the orientation and 〈P2〉 the alignment. The calculated moments have been compared with data available from different molecular beam experiments, without recourse to a specific collision model. For the passage of the molecules through different electric fields, the adiabatic approximation has been assumed to apply, except for curve crossings with extremely small gaps. For methylfluoride and methylchloride, the calculated orientations are in agreement with the measured steric effects of the reactive collisions...

Reactions of Ca and Sr with CH3X and CF3X (X Br, I). A laser-induced fluorescence study on nascent CaBr, SrBr, and SrI

Abstract Continuous-wave laser-induced fluorescence (LIF) spectra have been measured of nascent SrBr, SrI, and CaBr, formed in the reactions of Sr with CH 3 X and CF 3 X (X  Br,I), and of Ca with CH 3 Br and CF 3 Br. The bandhead spectra measured for the reactions involving Sr can be attributed to the formation of highly vibrationally excited SrX; for the reaction with CF 3 I vibrational bandheads up to ν ″ = 42 in SrI are observed. The LIF spectra of CaBr formed in the reactions of Ca with CH 3 Br and CF 3 Br can be interpreted as a sum of a thermal and a nonthermal spectrum. These two types of spectra originate from the formation of CaBr inside the Ca oven and in the reaction region outside the oven, respectively. In all cases studied, the spectroscopic constants reported in the literature from studies at low t; values need only minor modifications in order to yield a satisfactory fit of the measured spectra, even though products were formed in highly excited vibrational states. The results are compared with those of other studies of reactions of alkaline earth metals with halogenated methanes. A comparison of the energy disposal into the alkaline earth monohalide product as a function of the average available energy is hampered because of the uncertainty of the latter. In the present study the available energy, which is the sum of the internal energy of the molecular reactant, the collision energy, and the bond energies involved, is consistently too low as to give a satisfactory fit of the alkaline earth monohalide bandhead spectrum. The relatively high fraction of the available energy disposed into vibration of the diatomic product is indicative of an early energy release, such as takes place in an electron jump model. The variation of the disposal into vibrational energy of the formed diatom among the various systems is attributed to a variation of the harpooning radius and its magnitude relative to the interatomic distance of the diatomic product.

Vibrationally excited state spectroscopy of radicals in a supersonic plasma

Abstract A plasma source based on a multilayer discharge geometry in combination with a time-of-flight REMPI experiment is used to study rotationally cold spectra of highly excited vibrational states of mass selected radicals. The rovibrational state distributions upon discharge excitation are characterised for the example of NO for ground state vibrational levels up to v″=18. Whereas rotational temperatures are lower than 50 K, a vibrational temperature of Tvib=6700±700 K is found.

Direct spectroscopic determination of the degree of orientation of parity-selected NO

Abstract If a polar molecule can be selected in a rotational state of definite parity, subsequent orientation of the molecule in an electric field mixes opposite parity states. The degree of mixing reflects the degree of orientation. Therefore, the intensity ratio of spectral lines that correspond to transitions starting from the two parity states being mixed, forms a sensitive and accurate probe of the molecular orientation. If saturation of the spectral lines of interest is avoided, the absolute degree of orientation can be determined, without recourse to other experimental data but line intensities. The method is illustrated for the case of the NO molecule.

Vibrational corrections of dipolar couplings: methylfluoride and 1,1-difluoroethene

Abstract Previously calculated vibrational corrections for methylfluoride are discussed and harmonic corrections are applied to 1,1-difluoroethane. For both molecules, vibrational motions do not account for inconsistencies between the measured anisotropic couplings and the geometry.

Brute force orientation and the two-dimensional oscillator

Abstract It has been shown by Loesch and Remscheid, and by Friedrich and Hershbach, that applying a strong, homogeneous electric field is a versatile method for orienting polar molecules in a molecular beam. It is convenient to expand the orientation distribution function in Legendre polynomials. The Legendre moments can be calculated from the expansion of the wavefunctions in free-rotor functions, with different J, by carrying out the appropriate integrations. Here, we use an alternative method based on a Clebsch-Gordan expansion, by which explicit integration is avoided. Early quantum-mechanical perturbation treatments of rotating polar molecules in an electric field are reviewed. For the average orientation of a linear dipole in a strong electric field a simple relation is obtained which is based on a zeroth-order perturbation treatment of vibration in two dimensions. This relation is useful for estimating the average orientation of rotationally cold, linear dipoles in a strong electric field.

High-resolution Laser Spectroscopy of NO2 just above the X2 A1-A2B conical intersection: Transitions of K_=1 stacks

The complexity of the absorption spectrum of NO2 can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of 2A1 symmetry and the first electronically excited state of 2B2 symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10 000 to 14 000 cm−1 above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest, rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12 658 cm−1, so that the fine- and hyperfin...

HIGH-RESOLUTION BOLOMETRIC SPECTROSCOPY OF NO2 IN THE REGION OF 13352 CM-1

Abstract The vibronic band at 13352 cm−1 of NO2 is measured up to the hyperfine structure by bolometric detection. The excited vibronic state involved is a hybrid state having a major contribution from electronic ground state. This follows from the hyperfine splitting of the K_=0 and K_=1 stacks as well as from the value of the rotational constant A′. The anomalous intensity distribution induced by rovibrational interactions is not observed in the vibronic band analysis here. Although we suggest the absence of rovibronic interactions of significant strength, we underline the presence of vibronic interactions in the vibronic band analysed here.

Laser-induced fluorescence studies of excited Sr reactions: II. Sr(3P1)+CH3F, C2H5F, C2H4F2

The vibrational and rotational energy distributions of ground state SrF(X 2Σ) formed in the reactions of electronically excited Sr(3P1) with methylfluoride, ethylfluoride, and 1,1-difluoroethane have been studied by laser-induced fluorescence. Although the reactions of ground state Sr with these reactants are exothermic, no SrF products are observed for those reactions in this study. The fraction of available energy disposed into the sum of rotational and vibrational energy of the SrF(X 2Σ) product is approximately the same for all three reactions, i.e., 40%. The reaction of Sr(3P1) with CH3F results in very low vibrational excitation in the SrF reaction product. The product vibration increases in going to C2H5F and C2H4F2. It is concluded that the alkyl group influences the energy disposal mechanism in these reactions, and some suggestions are given for a partial explanation of the observations.