Sub-Doppler Measurement of Excited-State Rotational Constants and Rotational Coherence by Picosecond Multiphoton Ionization Mass Spectrometry

A method is presented here for one-photon sub-Doppler measurement of excited-state rotational constants and coherence of large polyatomic molecules. The method, which relies on the concept of purely rotational coherence in molecules, utilizes (polarized) picosecond pump-probe multiphoton ionization (MPI) mass spectrometry. It offers improved temporal resolution (pulse width limited) and is applicable to weakly or nonfluorescing molecules. The present implementation in a molecular beam provides measurements of the rotational constants in the excited (S₁) state of trans-stilbene and gives information on the direction of the relevant transition moments involved. From the coherence decay of the initially prepared state we obtain the dephasing time, which we discuss in relation to experiments involving vibrational / rotational energy redistribution.