Juan Ortigoso

Time evolution of pendular states created by the interaction of molecular polarizability with a pulsed nonresonant laser field

Previous investigations have shown that the instantaneous eigenstates of a molecule interacting via its polarizability with a strong electric field of a nonresonant laser pulse are pendular hybrids of field-free rotational states, aligned along the field direction. However, nonadiabatic effects during the time evolution of the initial field-free rotational state could cause the molecule to end up in a state described by a linear combination of pendular states (a rotational wavepacket) whose alignment properties are not a priori known. We report a computational study of the time evolution of these states. We solve the reduced time-dependent Schrodinger equation for an effective Hamiltonian acting within the vibronic ground state. Our numerical results show that the time evolution and the achievement of adiabatic behavior depend critically on the detailed characteristics of the laser pulse and the rotational constant of the molecule.

The ISO/SWS Spectrum of IRC +10216: The Vibrational Bands of C2H2 and HCN*

We present the Infrared Space Observatory/Short-Wavelength Spectrometer full grating resolution spectrum of IRC +10216, which is dominated by strong absorption/emission bands of C2H2 and HCN. All C2H2 bands and the strong near-infrared stretching bands of HCN are observed in absorption, whereas the fundamental, hot, and combination bands of HCN involving the nu2 bending mode around 14 µm are observed in emission. Particularly strong is the HCN nu2=20-->nu2=11 vibrational transition at 14.3 µm. The most plausible mechanism for such emission is the radiative pumping of molecules from the ground to the nu2=20 state (7.1 µm) followed by radiative decay: nu2=20-->nu2=11. We present detailed models for HCN that verify the efficiency of the mentioned effect. The HCN abundance inferred from these models is &parl0;1.5-3&parr0;x10-5.

The ν2 and ν4 IR bands of SO3

Abstract The infrared spectrum of SO 3 has been recorded at 0.005 cm −1 resolution from 460 to 567 cm −1 . Over 2000 lines have been assigned to the Coriolis-interacting ν 2 and ν 4 fundamental bands. A rotational analysis has been performed which has allowed us to refine rovibrational parameters of the ground state and the v 2 = 1 and v 4 = 1 states. A discussion is presented on the different reductions of the Hamiltonian employed. Improved values are obtained for equilibrium parameters of this molecule.

The ν2 and ν3 bands and ground state constants of OClO

Abstract The Fourier Transform IR spectra of the ν 2 and ν 3 bands of ClO 2 (OClO) have been recorded at ∼0.005 cm −1 resolution. Both bands have been analyzed in terms of a rotation-spin-rotation Hamiltonian model, including up to sextic centrifugal distortion constants and spin-rotation centrifugal distortion parameters. More than 4300 transitions belonging to these bands have been assigned for either the 35 Cl or 37 Cl species. Accurate values for the band origins and the rotational and spin-rotation parameters of the v 2 = 1 and v 3 = 1 states have been obtained from this data set. Ground state combination differences from all three fundamental bands of this molecule, plus ground state microwave data, have been used to improve the ground state constants. A list with the assigned transitions of ν 3 and their calculated intensities is given.

High-resolution infrared spectrum of the ν1 band of OClO

Abstract The infrared absorption spectrum of the ν1 band of OClO (care must be taken to distinguish it from ClOO, which also exists) has been recorded in the 950 cm−1 region, with a Fourier transform infrared spectrometer with an instrumental resolution of ∼0.004 cm−1. Most lines appear as doublets owing to the spin-rotation interaction present in this molecule. Around 2800 lines have been assigned for the 35ClO2 species and ∼800 for the 37ClO2 species. In addition, a number of lines of the “hot band”, ν1 + ν2 − ν2, have been assigned for the 35ClO2 species. Effective rotational and spin-rotational spectroscopic constants have been obtained for the ground and the v1 = 1 vibrational states of 35ClO2 and 37ClO2 and the corresponding band origins have also been determined. Fermi resonance between the 2ν2 and ν1 bands has been found to be negligible; however, a weak resonance between the Ka = 7 levels of v1 = 1 and the Ka = 9 levels of v2 = 2 has been observed. The measured integrated ν1 band strength is 87 ± 6 cm−2 atm−1 at 300 K.

Porosity and band- strength measurements of multi-phase composite ices

We use experimental mid-infrared optical constants and extended effective medium approximations to determine the porosity and the band strengths of multi-phase composite ices grown at 30 K. A set of porous H2O:CH4 ices are taken as a prototypical example. As a benchmark and proof of concept, the stoichiometry of the ice constituents is retreived with good accuracy from the refractive indices and the extinction coefficients of the reference binary ice mixtures with known compositions. Accurate band strengths are then calculated from experimental mid-infrared spectra of complex ices. We notice that the presence of pores has only a small effect on the overall band strengths, whereas a water dilution can considerably alter them. Different levels of porosity are observed depending on the abundance of methane used as a gas contaminant premixed with water prior to background deposition. The absorption profiles are also found to vary with deposition rate. To explain this, we use Monte Carlo simulations and we observe that the deposition rate strongly affects the pore size distribution as well as the ice morphology through reorganization processes. Extrapolated to genuine interstellar ices, the methodology presented in this paper can be used to evaluate the porosity and to quantify the relative abundances from observational data.

The ν1 band of ketene

Infrared and Raman spectra of the ν1 band of ketene, H2CCO, have been recorded at Doppler resolution. The infrared spectrum has been obtained with a difference frequency infrared spectrometer, and the Raman spectrum of the Q branches of this band has been recorded using a stimulated Raman spectrometer. The vib‐rotational analysis of the data is very complicated because of many crossings with other vibrational states, which can interact with v1=1 through Fermi or Coriolis mechanisms. We present a discussion on global and local resonances, and we are able to extract information on the perturbing levels and on the perturbation parameters, even though the perturbing bands are not always detected in the spectrum.

Mechanism of molecular orientation by single-cycle pulses

Significant molecular orientation can be achieved by time-symmetric single-cycle pulses of zero area, in the THz region. We show that in spite of the existence of a combined time-space symmetry operation, not only large peak instantaneous orientations, but also nonzero time-average orientations, over a rotational period, can be obtained. We show that this unexpected phenomenon is due to interferences among eigenstates of the time-evolution operator, as was described previously for transport phenomena in quantum ratchets. This mechanism also works for appropriate sequences of identical pulses, spanning a rotational period. This fact can be used to obtain a net average molecular orientation regardless of the magnitude of the rotational constant.

THE K-ROTATIONAL LABELING PROBLEM FOR EIGENVECTORS FROM INTERNAL ROTOR CALCULATIONS : APPLICATION TO ENERGY LEVELS OF ACETALDEHYDE BELOW THE BARRIER

The problem of attaching K rotational quantum number labels to computer-generated numerical eigenvectors with extensive basis set mixing is considered for the internal-rotationoverall-rotation problem in molecules with one methyl top. Quantum number labeling problems arise physically because the torsional and the rotational degrees of freedom both pass from one limiting case to another as the torsional energy moves from below the top of the internal rotation barrier to above it, i.e., the torsional degree of freedom changes from a vibration to an internal rotation, while the rotational degree of freedom moves its direction of quantization from a principal axis to an axis depending also on angular momentum generated by the methyl top rotation. Since the choice of axis system, basis set, and computational scheme all influence the eigenfunction labeling procedure, consideration is limited to a commonly used two-step matrix-diagonalization scheme and to acetaldehyde as a numerical example. Torsional labels vt...

The internal axis system of molecules with one large amplitude internal motion

An old puzzle of theoretical molecular spectroscopy, the derivation of the internal axis system (IAS) of molecules with one large amplitude internal motion and asymmetric top and asymmetric frame has been solved by a simple and transparent method. The basic idea has been to consider the large-amplitude-coordinate-dependent IAS as the result of quantum mechanical evolution in ρ of an initial system of axes, where ρ denotes the coordinate describing the large amplitude motion. A special choice of the operator governing ρ evolution guarantees that the ρ evolved system is the IAS. Floquet theory provides both a simple analytical expression for the evolution matrix and a means to efficiently calculate this matrix. Numerical examples have revealed fast convergence of the method to the IAS.