Emission spectroscopy of H₂O dissociating in the B̃¹A₁ state: Rapid bending motion manifested through excitation of high bending states of H₂O (X̃)

We present a theoretical and experimental investigation of the emission spectrum of dissociating water after excitation in the second absorption band (X̃¹A₁→B̃¹A₁). The calculations are performed in the time-dependent wave packet formalism employing an ab initio potential energy surface. All three degrees of freedom (the two OH stretching modes and the HOH bending mode) are taken into account. The B̃¹A₁ potential energy surface depends strongly on the HOH bending angle which leads to very fast opening of this angle after the water molecule is promoted to the excited electronic state. As a consequence, we observe, both experimentally and theoretically, the excitation of high bending states in the X̃ ground state. According to the wave packet study the emission spectrum is determined in the first ten femtoseconds of the motion in the excited state. The agreement with the measured spectrum for an excitation wavelength of 141.2 nm is good.