Electronic continua in time-resolved photoelectron spectroscopy. II. Corresponding ionization correlations

Abstract

We investigate further the role of ion electronic continua in time-resolved photoelectron spectroscopic measurements of ultrafast nonadiabatic coupling. In the preceding paper ͓Blanchet, Zgierski, and Stolow, J. Chem. Phys. 114, 1194 ͑2000͔͒, the limiting case of complementary ionization correlations permitted a disentangling of electronic from vibrational dynamics. Here we examine the other limiting case in which the nonadiabatically coupled sates ͑e.g., S 2 and S 1 ͒ correlations correspond to the same ionic continua, presumably an unfavorable case. We use ultrafast internal conversion in the polyaromatic hydrocarbons phenanthrene and naphthalene as examples. In this situation, the geometry changes ͑displacements͒ upon nonadiabatic crossing and upon ionization will strongly affect the ability to disentangle electronic from vibrational dynamics. Particularly, phenanthrene and naphthalene are both very rigid molecules and have small displacements upon internal conversion and ionization, still allowing for direct monitoring of the S 2 state internal conversion rate.

Spectroscopy and dynamics of excited states in maleimide and N-methyl maleimide: Ionic projection and ab initio calculations

“…Assuming that the molecule retains C 2v symmetry upon excitation, and that the ionization step can be described within the Born-Oppenheimer approximation, these bands should be associated with ionization to totally symmetric vibrational levels in the ion. As Table IV shows, we find indeed that it is not difficult to assign the spectrum in terms of ionization to vibrational levels of D 0 that involve predominantly the ring deformation mode 8 ϩ and the CvC stretching vibration 4 ϩ . At the same time, it should be remarked that the resolution in the spectrum does not rule out other assignments as indicated as well for some of the entries in the same table.…”

“…They show that for ionization of S 4 to D 0 dominant activity is expected in the 8 ϩ ͑ring deformation͒, 7 ϩ ͑C-C stretch͒, and 4 ϩ (CvC stretch͒ modes. This agrees with the experiment insofar as 8 ϩ and 4 ϩ are concerned, but the large difference between predicted and experimental activity of 7 ϩ is distressing.…”

“…Even so, it is reasonable to expect that the active vibrations in the photoelectron spectra of these levels are those that are associated with the differences in the equilibrium geometries of the S 1 and D 0 states. A measure for their activities is given by the I͓( i ϩ ) 0 1 ͔/I͓0 0 0 ͔ ratios, which have been calculated in the same way and at the same level as described above for S 4 . The results given in Table V show that for S 1 an ionization behavior is expected that is qualitatively similar to that of S 4 .…”

“…A recent study of Stolow et al gives in this respect a good overview of what has been done up till now. 5 A useful concept that was introduced in the studies of Stolow et al was that of corresponding 4 and complementary 3 ionization correlations, i.e., the ionization behavior of coupled states that correlate with the same and with different ionic states, respectively.…”

“…26 There it was seen that a limited number of molecular orbitals are important for the description of the lower excited states of the neutral molecule. Preliminary ab initio calculations performed by us confirmed this picture: for maleimide of C 2v symmetry it was found that the relevant orbitals are the symmetric n o ϩ (a 1 ) and antisymmetric n o Ϫ (b 2 ) combinations of the lone pair orbitals on the oxygen atoms, a CvC bonding orbital (b 1 ) that we will label as CvC , a orbital (b 1 ) that has a dominant contribution from the nitrogen atom and is labeled as N , and an antibonding * orbital (a 2 4 . On the basis of their electronic description, we can anticipate that the equilibrium geometry and force fields of the states of interest (S 1 to S 4 ) are different.…”