Vibrationally Mediated Photodissociation in a Beam

Vibrationally Mediated Photodissociation (Gas Phase)

The corner lab is dedicated to photodissociation of molecules in the gas phase, both under cell (room temperature) and molecular beam (jet-cooled) conditions. We use laser induced fluorescence (LIF), time-of-flight mass spectrometry (TOFMS), and velocity map imaging to detect the fragment products and determine how vibrational excitation prior to photodissociation affects the molecule's transition to and subsequent evolution on the excited state surface.

Velocity Map Imaging
We have recently introduced velocity map imaging (VMI) to our detection schemes. In a photodissociative process, fragments fly out from the interaction region with translation energy determined by the difference between the energy of the photon of light and the energy required to form the photofragments. This creates a series of nested spheres, each containing information on the internal energy of the fragment states. Ionizing the fragments and crushing these spheres on a detector allows detection of all these states. VMI uses an open electrode scheme that avoids the blurring caused by a finite interaction volume. Instead, fragments formed in different spatial locations but with the same velocity are mapped to the same pointon the detector. Integration around each ring from a center cut through the spheres produces an energy distribution. Angular distributions, which will give information on the type of transition and lifetime of the dissociating state, can also be determined.

ViMP of Methanol (CH₃OH)
We have detected H atom fragments from ViMP of methanol (CH₃OH) in a molecular beam¹. In this experiment, the pump laser excites the molecules with one quanta of vibrational excitation in the OH stretch prior to photodissociation and ionization

Figure 1: electronic action spectra of vibrationally excited methanol and one-photon absorption spectrum of ground state methanol.¹.

of H atoms with REMPI probe photons (243.1nm). The total electronic excitation energy (pump and photolysis) spectrum from the vibrationally excited methanol begins about 2600 cm^-1 lower in energy than from the ground state (Figure 1). It also exhibits a sharp decrease around 54,000 cm^-1 where the ground state is at a maximum. These trends are explained by considering the overlap of the vibrational state wavefunction with the excited state dissociative wavefunction. The shift to lower energy is the result of a wavefunction that extends farther along the dissociation coordinate. The decrease reflects the nodal structure of the vibrational wavefunction.

ViMP of Ammonia
The first singlet excited state of ammonia is a model system to study nonadiabatic photodissociation dynamics. The A¹A₂ state of ammonia is quasibound with a barrier in the exit channel leading to products H + NH₂ (Figure 2). The competition between nonadiabatic and adiabatic dissociation governs the branching between the ground and first electronic excited state of the NH₂ fragment in the region beyond the barrier on the excited state. An adiabatic dissociation is a trajectory that bypasses the conical intersection (intersection of the ground and electronic state) and preferentially stays on the upper adiabatic surface, forming excited state NH₂(²A₁) and H products, while a nonadiabatic dissociation is a trajectory that passes through the funnel of the conical intersetion, forming ground electronic state NH₂(²B₁) + H.

Figure 2: Cut through A and X state potential energy surfaces of ammonia².

We have obtained the vibronic spectra of the A state ViMP of jet-cooled ammonia. We were able to identify resonances of specific vibrational modes, such as the symmetric and antisymmetric N-H stretch in the excited electronic state. We then used these identifications to probe the excited electronic state. We have used Doppler spectroscopy combined with TOFMS of H atoms to determine which vibrational modes are correlated to adiabatic or nonadiabatic dissociation². In Figure 3, the Doppler profiles and speed distributions for the symmetric (1¹) stretch and the antisymmetric (3¹) stretch are shown. For both rovibrational transitions, the total energy is 48,700 cm^-1. However, the Doppler profiles and speed distributions for the symmetric and antisymmetric stretch are very different, suggesting that these vibrational modes correlate to different dissociation pathways. The speed distribution and Doppler profile is broader for the symmetric stretch, which means there exists a large fraction of fast hydrogen atoms available to participate in direct nonadiabatic dissociation through the conical intersection. The antisymmetric stretch has a narrow Doppler profile and speed distribution consisting of slow hydrogen atoms. Dissociation from this state is dominated by the adiabatic pathway, bypassing the conical intersection and preferentially staying on the upper adiabatic surface.

Figure 3: Doppler profiles (above) and speed distributions (below) for dissociation from the symmetric and antisymmetric stretch of A state ammonia².

References
1. J. M. Hutchison, R. J. Holiday, A. Bach, S. Hsieh, and F. F. Crim, J. Phys. Chem. A. 108, 8115 (2004)
2. A. Bach, J. M. Hutchison, R. J. Holiday, and f. F. Crim, J. Phys. Chem. A. 107, 10490 (2003).; A. Bach, J. M. Hutchison, R. J. Holiday, and F. F. Crim, J. Chem. Phys. 116, 9315 (2002)

People in this lab:

Heidi
Amanda