The photoinduced ring-opening reaction of 1,3-cyclohexadiene (CHD) to produce 1,3,5-hexatriene (HT) plays an essential role in the photobiological synthesis of vitamin D3 in the skin. This reaction follows the Woodward–Hoffmann rule, and C5–C6 bond rupture via an electronically excited state occurs with conrotatory motion of the end CH2 groups. However, it is noted that the photoexcited S1(π,π*) state of CHD is not electronically correlated with the ground state of HT, and the reaction must proceed via nonadiabatic transitions. In the present study, we have clearly observed the nonadiabatic reaction pathway via the doubly excited state of CHD using ultrafast extreme UV photoelectron spectroscopy. The results indicate that the reaction occurs in only 68 fs and creates product vibrational coherence. Extensive computational simulations support the interpretation of experimental results and provide further insights into the electronic dynamics in this paradigmatic electrocyclic ring-opening reaction.

Ultrafast electronic relaxation following 9.3 eV photoexcitation of gaseous acetone was investigated with time-resolved photoelectron imaging spectroscopy. An intense photoionization signal due to a transition from the 4 1 A 1 (π,π*) state to the D 1 (π −1 ) cationic state diminishes within 50 fs, owing to vibrational wave packet motion leaving our observation energy window. Additional photoionization signals were assigned to transitions from Rydberg states with principal quantum numbers of 3−8 to the D 0 (n −1 ) cationic state, created by strong vibronic couplings with the bright 4 1 A 1 (π,π*) state. The deactivation processes of the 4 1 A 1 (π,π*) and Rydberg states are discussed based on their decay profiles obtained from a time−energy map of photoelectron kinetic energy distributions.

We present photoelectron spectroscopy of liquid films generated in a vacuum with microfluidic devices based on liquid-liquid or gas-liquid collisions. The results are compared to those for a standard liquid microjet technique.

The photoinduced ring-opening reaction of 1,3-cyclohexadiene to produce 1,3,5-hexatriene is a well-known example of the Woodward-Hoffmann rule for stereochemical reactions governed by molecular orbital symmetry, and it plays an essential role in photobiological synthesis of vitamin D3 in the skin. Since the photoexcited 11B state of 1,3-cyclohexadiene is not electronically correlated to the ground state of 1,3,5-hexatriene, the reaction is expected to proceed via non-adiabatic transitions through a doubly excited 21A electronic state. However, spectroscopic observation of this elusive state has been difficult. Here we present the results of a photoelectron spectroscopy study using table-top ultrafast deep and extreme UV lasers, based on filamentation four-wave mixing and high-harmonic generation, which enabled us to arrest the 21A state. It is shown that the ring-opening reaction takes only 60 fs to complete, which is a considerably shorter time than previous experimental and theoretical estimates. The ballistic reaction creates vibrational coherence in the reaction products.