Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations.

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D1 surface with little excess energy at the equilibrium neutral geometry. These D1 ions are preferably formed because in this case D1 and D0 switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D1 surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D1/D0 conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D0 becomes populated with molecules that are energized mainly along the cis-trans isomerization coordinate, with the kinetic energy above the cis-trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D0 surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole-lone pair interaction that drives the molecule along the cis-trans isomerization coordinate to facilitate the transition between the involved electronic states.

Controlling dissociation of alkyl phenyl ketone radical cations in the strong-field regime through hydroxyl substitution position.

The hydroxy-substituted alkyl phenyl ketones 2'-, 3'- and 4'- (ortho, meta, and para) hydroxyacetophenone were excited in the strong-field regime with wavelengths ranging from 1200-1500 nm to produce the respective radical cations. For 2'- and 3'-hydroxyacetophenone, the parent molecular ion dominated the mass spectrum, and the intensity of the fragment ions remained unchanged as a function of excitation wavelength. In contrast, 4'-hydroxyacetophenone exhibited depletion of the parent molecular ion with corresponding enhanced formation of the benzoyl fragment ion upon excitation with 1370 nm as compared with other excitation wavelengths. Density functional (DFT) calculations suggest that dissociation occurs when the acetyl group in 4'-hydroxyacetophenone radical cation twists out-of-plane with respect to the phenyl ring, enabling a one-photon transition between the ground cation state D0 and the excited cation state D2 to occur. The DFT calculations also suggest that the lack of dissociation in the wavelength-resolved strong-field excitation measurements for 2'- and 3'-hydroxyacetophenone arises because both isomers have a barrier to rotation about the carbon-carbon bond connecting the phenyl and acetyl groups. These results help elucidate the effects of substituents on the torsional motion of radical cations and illustrate the potential for controlling molecular dissociation through the addition of substituents.

Measurement of ionic resonances in alkyl phenyl ketone cations via infrared strong field mass spectrometry.

Strong-field excitation of alkyl phenyl ketone molecules reveals an electronic resonance at 1370 nm in the radical cations upon measuring mass spectra as a function of excitation wavelength from 1240 to 1550 nm. The ratio of the benzoyl fragment ion to parent ion signal in acetophenone increases from 1:1.5 at 1240 nm excitation to 5:1 at 1370 nm (0.9 eV), and back to 1:1 at 1450 nm. Unlike acetophenone and propiophenone, the homologous molecules acetone and ethylbenzene exhibit no wavelength-dependent fragmentation patterns over the range from 1240 to 1550 nm, supporting the hypothesis that the electronic structure of the alkyl phenyl ketone cation enables the one-photon transition. Calculations on the acetophenone and propiophenone radical cations show the existence of a bright state, D2, 0.87 and 0.88 eV, respectively, above the ground-state D0 minimum. Calculations of the potential energy surfaces of the acetophenone radical cation suggest that a D2 → D0 radiationless transition precedes dissociation on D0. Upon population transfer to the D2 surface, the wavepacket motion is directed toward a three-state conical intersection (D0/D1/D2) that facilitates the photodissociation by converting electronic to vibrational energy on the D0 surface.

Strong Field Adiabatic Ionization Prepares a Launch State for Coherent Control.

We demonstrate that excitation of acetophenone with a strong field, near-infrared femtosecond pulse (1150-1500 nm) results in adiabatic ionization, producing acetophenone radical cation in the ground electronic state. The time-resolved transients of the parent and fragment ions probed with a weak 790 nm pulse reveal an order of magnitude enhancement of the peak-to-peak amplitude oscillations, ∼ 100 fs longer coherence time, and an order of magnitude increase in the ratio of parent to fragment ions in comparison with nonadiabatic ionization with a strong field 790 nm pulse. Equation of motion coupled cluster and classical wavepacket trajectory calculations support the mechanism wherein the probe pulse excites a wavepacket on the ground surface D0 to the excited D2 surface at a delay of 325 fs, resulting in dissociation to the benzoyl ion. Direct population transfer to the D2 state within the duration of a 1370 nm pump pulse eliminates wavepacket oscillation on the D0 state.

Measurement of an Electronic Resonance in a Ground-State, Gas-Phase Acetophenone Cation via Strong-Field Mass Spectrometry.

A one-photon ionic resonance is measured in the strong-field regime in acetophenone by recording the mass spectra as a function of excitation wavelength from 800 to 1500 nm. The ratio of the benzoyl to parent ion signals in the mass spectrum varies significantly with excitation wavelength, where the highest ratio observed upon excitation at 1370 nm (0.90 eV) indicates the presence of a one-photon resonance. At the resonant wavelength, the ratio of the benzoyl to parent ion signals increases linearly with laser intensity over a range from 1.1 × 10(13) to 6.0 × 10(13) W cm(-2). The ratio increases by a factor of 5 at 1370 nm with increasing pulse duration from 60 to 100 fs. Calculations using the equation of motion coupled cluster method support the existence of a one-photon transition from the ground ionic to a dissociative excited ionic state (0.87 eV), where motion of the acetyl group from a planar to nonplanar structure within the pulse duration enables the otherwise forbidden transition.