Monitoring ultrafast vibrational dynamics of isotopic molecules with frequency modulation of high-order harmonics

Lixin He,Muzi Li,André D Bandrauk,Chunyang Zhai,Pengfei Lan,Xue-Bin Bian,Wenjing Shi,Peixiang Lu,Qingbin Zhang,Feng Wang,Xiaosong Zhu,Wei Cao

doi:10.1038/s41467-018-03568-3

Abstract

Molecules constituted by different isotopes are different in vibrational modes, making it possible to elucidate the mechanism of a chemical reaction via the kinetic isotope effect. However, the real-time observation of the vibrational motion of isotopic nuclei in molecules is still challenging due to its ultrashort time scale. Here we demonstrate a method to monitor the nuclear vibration of isotopic molecules with the frequency modulation of high-order harmonic generation (HHG) during the laser-molecule interaction. In the proof-of-principle experiment, we report a red shift in HHG from H2 and D2. The red shift is ascribed to dominant HHG from the stretched isotopic molecules at the trailing edge of the laser pulse. By utilizing the observed frequency shift, the laser-driven nuclear vibrations of H2 and D2 are retrieved. These findings pave an accessible route toward monitoring the ultrafast nuclear dynamics and even tracing a chemical reaction in real time.

Highlights

Molecules constituted by different isotopes are different in vibrational modes, making it possible to elucidate the mechanism of a chemical reaction via the kinetic isotope effect
For isotopic molecules, the vibrational modes depend sensitively on the masses of its constituent isotopic atoms, which provides an important method to determine the mechanism of a chemical reaction via the kinetic isotope effect[3,4], namely, the fact that heavier isotopes tend to react more slowly than lighter ones
We experimentally observed the red shift in high-order harmonic generation (HHG) from isotopic molecules H2 and D2

Summary

Introduction

Molecules constituted by different isotopes are different in vibrational modes, making it possible to elucidate the mechanism of a chemical reaction via the kinetic isotope effect. Many works have been carried out to investigate the effects of nuclear motion in strong-field ionization[18,19,20,21] and molecular high-order harmonic generation (MHOHG)[22,23,24,25,26]. By analyzing the AMs in high-order harmonic generation (HHG) from isotopic molecules (H2 and D2), the intracycle nuclear dynamics has been theoretically predicted[27] and experimentally detected[28,29]. This method is restricted because the propagation and other inherent physical factors, such as two-center interference[27,30] and energy-dependent rescattering cross sections[31], may affect the harmonic intensity. From the observed frequency shift, the nuclear motions of H2 and D2 are successfully retrieved, which agree well with the calculations from nonBorn–Oppenheimer time-dependent Schrödinger equation (NBO–TDSE)

Methods

Results

Conclusion