Photoionization and Dissociative Photoionization of Acetaldehyde in the 10.0-13.7 eV Range by Synchrotron Photoelectron Photoion Coincidence Spectroscopy.

Photoionization and dissociative photoionization of acetaldehyde (CH 3 CHO) in the 10.0-13.7 eV energy range are studied by using synchrotron radiation double imaging photoelectron photoion coincidence spectroscopy (i 2 PEPICO). The X 2 A' and A 2 A" electronic states of CH 3 CHO + as well as the Franck-Condon gap region between these two states have been populated with several vibrational sequences and assigned in the high-resolution slow photoelectron spectrum (SPES). The adiabatic ionization energies (AIEs) of the X 2 A' and A 2 A" states are measured at 10.228±0.006 and 12.52±0.05 eV, respectively. The present results show that the X 2 A' state is a stable state while the A 2 A" state is fully dissociative to produce CH 3 CO + , CHO + and CH 4 + fragment ions. The 0 K appearance energies (AE 0K ) of CH 3 CO + and CHO + fragment ions are determined through the modeling of the breakdown diagram, i. e., AE 0K (CH 3 CO + )=10.89±0.01 eV (including a reverse barrier of ~0.19 eV) and AE 0K (CHO + )=11.54±0.05 eV. In addition, the dissociation mechanisms of CH 3 CHO + including statistical dissociation, direct bond breaking and isomerization are discussed with the support of the calculated dissociation limits and transition state energies.