Julio Santos

Vibrational–torsional coupling. High‐resolution stimulated Raman spectrum of the ν3 band of ethane (12C2H6)

We report a stimulated Raman spectrum of the Q branch of the ν3 band of ethane recorded with a resolution of about 0.0055 cm−1 and an absolute wave‐number accuracy of the order of 0.001 cm−1. This spectrum is analyzed in terms of a vibrational–torsional coupling Hamiltonian that allows for the calculation of the observed wave numbers within the experimental uncertainty. The following spectroscopic parameters (in cm−1) have been obtained: ν30=994.1108(10), B3A1s−B0=−0.006 165(3), B3E3d−B0=−0.006 170(2), B3E3s−B0 =−0.006 184(2), B3A3d−B0=−0.006 257(2), A3A1s−A0=−0.003 037(15), A3E3d−A0 =−0.003 040(12), A3E3s−A0=−0.003 054(12), and A3A3d−A0=−0.003 065(12). A strong dependence of the threefold torsional potential barrier with respect to the q3 normal mode, ∂V3/∂q3=−275.9±1.4 cm−1, is deduced from the present analysis.

Performance of a solenoid‐driven pulsed molecular‐beam source

The characteristics of a commonly used pulsed valve for the production of free jets and molecular beams are analyzed in detail. Special attention is paid to the formation of gas pulses providing a quasisteady flow during a certain time interval within the pulse duration, and to the estimation of a scaling parameter (effective diameter) for the description of the flow field. The adequacy of this effective diameter is checked by performing time‐of‐flight measurements on molecular beams of Ne, N2, and CH4, and stimulated Raman spectra on free jets of N2 and CH4.

High resolution Raman spectroscopy from vibrationally excited states populated by a stimulated Raman process: 2ν1−ν1 of CH412

A new pump and probe technique for investigating vibrationally excited states via high resolution Raman spectroscopy is proposed. In a first step, a vibrational state, typically not accessible by a dipole moment transition from the ground state (IR forbidden, Raman allowed transition), is populated in a doubly pulsed stimulated Raman process. This state is long lived as the radiative decay to the ground state is highly inefficient. After a delay of several nanoseconds, the high resolution spectrum corresponding to a transition from this excited level is recorded. The second process follows the quasi-cw stimulated Raman spectroscopy scheme reported previously. Experimental aspects are discussed and the spectrum of 2ν1−ν1 of 12CH4 is presented as an illustration of the technique. The analysis of this spectrum has been done in two ways: first, by fitting simultaneously the ν1 and 2ν1 bands using an isolated band model; second, using a model based on the tetradecad-pentad system. The standard deviations of bo...

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.

Double modulation‐high resolution infrared spectroscopic technique: The ν3 band of the CH3 radical and excited states of CH4 in a hollow cathode discharge

In this work several lines of the ν3 fundamental band of the CH3 radical have been detected in a refrigerated hollow cathode discharge multipass cell filled with CH4, by using a discharge and laser amplitude double modulation technique, with a difference frequency laser spectrometer and a double phase sensitive detection. Bands of the precursor CH4 and of C2H4 and C2H6 produced in the discharge have been observed too. By Doppler broadening and absorption variations, the increase of temperature with the establishment of the discharge has been tested. It is only ∼15 °C. Nevertheless, a very high sensitivity in transmittance variations with temperature is achieved for transitions from highly excited levels of stable species. The method proposed can be very useful for the assignment of very weak bands.

Stimulated Raman spectra of jet-cooled ethylene

Abstract We have recorded stimulated Raman spectra of the central region of the v 1 band of ethylene, both in a jet at temperatures down to ≈25 K, and in a static cell at room temperature and at ≈173 K. The rotational temperatures of the jet are estimated by computer simulation of the observed spectra. Only Q branches are recorded in this experiment. A vib-rotational analysis is performed and improved values are obtained for the band origin and the rotational constants of the v 1 =1 state of this molecule. Some perturbations are found to affect the rotational levels of this state.

High-resolution q-cw SRS spectrum of 12CH4 in the region of the level crossing between ν1 and ν2 + ν4

Abstract Wavenumbers and relative peak heights of 62 peaks observed in the high-resolution “quasi-continuous wave Stimulated Raman Scattering” (q-cw SRS) spectrum of the 12CH4 molecule are reported for the first time. These peaks are assigned mainly to transitions with J values between 10 and 17 of the Q branch of ν1 and ν2 + ν4 bands. A comparison with the predictions of some theoretical models and some available IR data is made.

Rho-axis-system Hamiltonian for molecules with one large amplitude internal motion

The rho-axis system and the rho-axis-system Hamiltonian were devised and formulated to describe the rotational–internal–rotational motion of molecules possessing certain local geometrical symmetries. Nevertheless, they were employed to molecules without the required symmetries and apparently lying outside the range of their applicability. To justify such applications and to facilitate first-principles calculation of the corresponding spectroscopic parameters, the rho-axis-system and the rho-axis system Hamiltonian must be derived without assuming or imposing symmetry constraints. A derivation satisfying this requirement is described, and the rho-axis method is extended to molecules having a large amplitude internal motion of any kind.

Tailored particle current in an optical lattice by a weak time-symmetric harmonic potential

Quantum ratchets exhibit asymptotic currents when driven by a time-periodic potential of zero mean if the proper spatiotemporal symmetries are broken. Recently, there has been debate on whether directed currents may arise for potentials which do not break these symmetries. We show here that in the presence of degeneracies in the quasienergy spectrum, long-lasting directed currents can be induced, even if the time-reversal symmetry is not broken. Our model can be realized with ultracold atoms in optical lattices in the tight-binding regime, and we show that the time scale of the average current can be controlled by extremely weak fields.

Long-lasting molecular alignment: Fact or fiction?

It has been suggested that appropriate periodic sequences of laser pulses can maintain molecular alignment for arbitrarily long times [J. Ortigoso, Phys. Rev. Lett. 93, 073001 (2004)]. These aligned states are found among the cyclic eigenstates of truncated matrix representations of the one-period time propagator U(T,0). However, long time localization of periodic driven systems depends on the nature of the spectrum of their exact propagator; if it is continuous, eigenstates of finite-basis propagators cease to be cyclic, in the long time limit, under the exact time evolution. We show that, for very weak laser intensities, the evolution operator of the system has a point spectrum for most laser frequencies, but for the laser powers needed to create aligned wave packets it is unknown if U(T,0) has a point spectrum or a singular continuous spectrum. For this regime, we obtain error bounds on the exact time evolution of rotational wave packets that allow us to determine that truncated aligned cyclic states do not lose their alignment for millions of rotational periods when they evolve under the action of the exact time propagator.