On the photoabsorption spectroscopy of water

The first photoabsorption band of water around 8 eV:is studied with the mol ecular dynamics computer simulation technique under ambient and supercritic al conditions. By employment of the polarizable TAB/ 10D potential model (B ursulaya, B. D.; Jeon,J.; Zichi,;D. A.; Kim, H. J. J. Chem. Phys. 1997, 108 , 3286), the electronic structure variations of the ground and low-lying ex cited states of water with solvation are explicitly accounted for via the t runcated adiabatic -basis-set representation. Compared with the (X) over ti lde(1)A(1) --> (A) over tilde(1)B(1) transition in vacuum, the absorption s pectrum;tends:to be blue-shifted and broadened in solution, consonant with experiments. It is found that both the solvation and Rydbergization destabi lizations of the first excited states of individual water molecules, compar ed with their respective ground states, contribute to the blue shift, while the line broadening arises from the inhomogeneous distribution of their so lvation environments. Also there is significant electronic mixing of the ex cited states, induced by solvation. As a result, their dipole character var ies dramatically with the fluctuating solvation environment as well as with the thermodynamic conditions, e.g., density. Its potential consequences fo r the Urbach tail of the spectrum observed in condensed phases are discusse d. Also new insights into multiphoton ionization of water from the excited- state electronic perspective are presented.