Interrogating nonadiabatic molecular dynamics using ultrafast nonlinear optics

Abstract

The field of femtochemistry seeks to comprehend the fundamental underlying mechanisms of the interaction between light and molecules and to study the ultrafast timescales on which these processes occur. In particular, the photoresistance of biologically relevant molecules to potential damage caused by absorption of ultraviolet radiation is of great interest. The so called “building blocks” of life use ultrafast non-radiative relaxation pathways for the dissipation of the high excess UV energy as vibrational energy into the surroundings, which is the key of their photoprotective function. The use of a “bottom-up” methodology for such investigations is applied to understand basic model UV chromophores, which are molecular sub-units of various bio-molecules such as, for example, the DNA bases and the melanin pigmentation system. The photophysics of the basic model chromophores, indole and the aniline derivatives N,N-dimethylaniline and 3,5-dimethylaniline, were investigated in the gas phase to understand the link between their molecular structure, the ultrafast non-adiabatic dynamics and thus their potential photoprotection function. This study was done with the powerful time-resolved photoelectron imaging (TRPEI) technique, which provides temporal, energy- and angle-resolved information related to the non-adiabatic relaxation dynamics operating within each molecular system. TRPEI is a highly differential pump-probe spectroscopic technique providing a detailed picture of the underlying processes, since it is sensitive to both electronic and nuclear motion within the molecule. The observation of the complete dynamical process using the TRPEI method is however restricted by the energy of the utilised probe pulse. For this reason the spectroscopic technique was improved with the integration of a newly built femtosecond vacuum ultraviolet (VUV) light source. The VUV laser pulses are generated in a four wave frequency mixing process in third order nonlinear media, such as noble gases. First results from this instrument are presented for the butadiene molecule. The combination of the new VUV laser light and the already powerful spectroscopic technique enables, in principle, the detection of the complete non-radiative relaxation process of a large variety of molecular systems.