Quantum Resonances in Selective Rotational Excitation of Molecules with a Sequence of Ultrashort Laser Pulses

Abstract

We experimentally investigate the effect of quantum resonance in the rotational excitation of the simplest quantum rotor--a diatomic molecule. Using the techniques of high-resolution femtosecond pulse shaping and rotational state-resolved detection, we measure directly the amount of energy absorbed by molecules interacting with a periodic train of laser pulses, and study their dependence on the train period. We show that the energy transfer is significantly enhanced at quantum resonance, and use this effect to demonstrate selective rotational excitation of two nitrogen isotopologs, (14)N(2) and (15)N(2). Moreover, by tuning the period of the pulse train in the vicinity of a fractional quantum resonance, we achieve selective rotational excitation of para- and ortho-isomers of (15)N(2).

Observation of nondispersing classical-like molecular rotation

“…Although the popular non-adiabatic approach to molecular alignment with intense non-resonant laser pulses can produce very narrow angular distributions [15][16][17][18][19][20][21][22], the angular momentum of the aligned molecules is usually poorly defined, both in magnitude and direction. Hence, the created rotational wave packet quickly disperses in angle, undergoing oscillations between aligned and non-aligned distributions.…”

Unified parameter for localization in isotope-selective rotational excitation of diatomic molecules using a train of optical pulses

“…Using a train of optical pulses, we can impart the isotope selectivity in rotational excitation, because the classical rotational period depends on the mass of the molecules [23][24][25][26][27]. The isotope selective population displacement of rotationally resonant terahertz pulses by the train is particularly considered to be practical [28][29][30].…”

Localization in Rotational Excitation of Diatomic Molecules Induced by a Train of Optical Pulses

“…This effect is called quantum resonance [2]. We can select one isotope from an isotopic mixture by tuning the pulse interval, as the period of molecular rotation depends on each isotope [3][4][5]. In the condition of ideal quantum resonance, the molecules at any initial rotational state will be rotationally excited and de-excited, and eventually they all will be transferred into highly rotationally excited states [6].…”