Rotational molecular dynamics of laser-manipulated bromotrifluoromethane studied by x-ray absorption

Abstract

We present a computational study of the rotational molecular dynamics of bromotrifluoromethane (CF(3)Br) molecules in gas phase. The rotation is manipulated with an off-resonant 800 nm laser. The molecules are treated as rigid rotors. Frequently, we use a computationally efficient linear rotor model for CF(3)Br, which we compare with selected results for full symmetric-rotor computations. The expectation value (cos(2) theta)(t) is discussed. Especially, the transition from impulsive to adiabatic alignment, the temperature dependence of the maximally achievable alignment, and its intensity dependence are investigated. In a next step, we examine resonant x-ray absorption as an accurate tool to study laser manipulation of molecular rotation. Specifically, we investigate the impact of the x-ray pulse duration on the signal (particularly its temporal resolution) and study the temperature dependence of the achievable absorption. Most importantly, we demonstrated that using picosecond x-ray pulses, one can accurately measure the expectation value (cos(2) theta)(t) for impulsively aligned CF(3)Br molecules. We point out that a control of the rotational dynamics opens up a novel way to imprint shapes onto long x-ray pulses on a picosecond time scale. For our computations, we determine the dynamic polarizability tensor of CF(3)Br using ab initio molecular linear-response theory in conjunction with wave function models of increasing sophistication: Coupled-cluster singles (CCS), second-order approximate coupled-cluster singles and doubles (CC2), and coupled-cluster singles and doubles (CCSD).

Computational studies of x-ray scattering from three-dimensionally-aligned asymmetric-top molecules

“…At a rotational temperature of T = 10 mK, almost all naphthalene molecules are in the rotational ground state. The rotational period of naphthalene at such a low temperature, τ rot ≈ 1/(A + B) ≈ 476 ps, is of the order of τ L = 100 ps, suggesting that the alignment dynamics are quasiadiabatic [55]. This is consistent with the observation that the overall alignment follows the laser pulse shape and, in addition, clear nonadiabatic features (fast oscillations) are visible.…”

X-ray refractive index of laser-dressed atoms

“…The ability to create almost arbitrarily shaped x-ray pulses opens up perspectives for the control of the dynamics of the inner-shell electrons in atoms. This way of shaping x-ray pulses is similar to our recent study of x-ray absorption by laser-aligned molecules where x-ray pulses are shaped by controlling the molecular alignment [10]. As the rotational dynamics of molecules takes place on a picosecond time scale, the x-ray pulse shaping is, however, done on a much slower time scale.…”

“…Recently, we studied molecules exposed to a laser with an intensity close to but still below the excitation and ionization threshold. If the molecule has an anisotropic polarizability tensor, they may be aligned along the linear laser polarization axis [6,8,9,10,11]; the x rays serve as an in situ probe of the rotational molecular dynamics. Going a bit higher in laser intensity, without exciting and ionizing, is possible for rare gas atoms.…”

1,4-Diazabicyclo-[2,2,2]-octane (DABCO)

“…The two photon ionisation scheme is also disadvantaged by a high contribution of the multiphoton ionisation signal to the measurements, which can add some complications to the interpretation of the data. The rotation period of a molecule depends on the rotational quantum number, J, and can be estimated from the rotational constant according to the following formula [134]:…”