Theoretical investigations on the vibronic coupling between the electronic states S0 and S1 of formic acid including the photodissociation at 248 nm

We studied the vibronic coupling of the S1 ← S0 transition in HCOOH and obtained partial and total optical oscillator strengths (OOS). Vibronic coupling is responsible for an increase of the total OOS of almost 50%. We confirmed theoretically that this diffuse absorption band corresponding to a n → π* transition has the major portion of the vibronic transition intensity associated with the CO stretching and CH bendings normal modes. We also discussed the implications of the vibronic coupling results for internal conversion process between the two states followed by molecular photodissociation due to photon absorption at 248 nm.