Femtosecond wave-packet revivals in ozone

Photodissociation of ozone following absorption of biologically harmful solar ultraviolet radiation is the key mechanism for the life protecting properties of the atmospheric ozone layer. Even though ozone photolysis is described successfully by post-Hartree-Fock theory, it has evaded direct experimental access so far, due to the unavailability of intense ultrashort deep ultraviolet radiation sources. The rapidity of ozone photolysis with predicted values of a few tens of femtoseconds renders both ultrashort pump and probe pulses indispensable to capture this manifestation of ultrafast chemistry. Here, we present the observation of femtosecond time-scale electronic and nuclear dynamics of ozone triggered by ∼10-fs, ∼2-µJ deep ultraviolet pulses and, in contrast to conventional attochemistry experiments, probed by extreme ultraviolet isolated pulses. An electronic wave packet is first created. We follow the splitting of the excited B-state related nuclear wave packet into a path leading to molecular fragmentation and an oscillating path, revolving around the Franck-Condon point with 22-fs wave-packet revival time. Full quantum-mechanical ab initio multiconfigurational time-dependent Hartree simulations support this interpretation.

Figure 1

(a) Top view and (b) side view of the differential time-dependent one-electron reduced density between the excited $B$ state and the ground $X$ state of ozone at the Franck-Condon point. After interaction of the ozone molecule with a ∼10-fs, ∼2-µJ DUV pulse, as used in the present work, the electron cloud starts oscillating between the two bonds with a periodicity of only 0.9 fs. This is equivalent to an alternating excess or lack of electrons on either side of the ozone molecule and is considered a precursor of the ozone photodissociation. Oxygen atoms of the ozone molecule (red spheres). Lack of electronic density (light blue surface isovalue: −0.0001 a.u.); excess of electronic density (light purple surface isovalue: +0.0001 a.u.). The moment of maximum $B$-state excitation coincides with 0 fs. Electronic charge densities for times greater than 20.1 fs originate from a coherence revival associated with the $B$-state PES dynamics, as discussed in the main text.

Figure 2

Sketch of the $X$ state (black solid line) and $B$ state (red solid line) related PESs of ozone as a function of the dissociative degree of freedom, $R_1$. The vibrational, $R_2$, and bending, α, degrees of freedom were kept constant ($R_2=2.42\mathrm{a}.\mathrm{u}.$; $\alpha =116.9^{\circ }$). The NWP in the ground state (bottommost Gaussian) is promoted from the ground $X$ state into the excited $B$ state (vertical blue arrow; uppermost Gaussian) at the FC point (green dot) and starts moving down the slope of the $B$-state PES, continuously probed by an XUV probe pulse (vertical purple arrows indicate photoionization at different delay times during the evolution of the molecular dynamics). Midway between the FC point (equilibrium position at $R_1=2.42\mathrm{a}.\mathrm{u}.$) and the outer range of the PES the NWP bifurcates where part of it returns to the FC point while the other part dissociates (rightmost Gaussian). It should be noted that the superposition of the two electronic $X$ and $B$ states can be considered as an oscillating electronic wave packet, since the $B$-state component contributes a very fast oscillating phase with regard to the $X$ state component (see Fig. 1).

Figure 3

(a) Sketch of the DUV generating type-I SHG setup as used in the experiment along with the pump-–probe delay line in one arm of the Mach-Zehnder interferometer: The spectrally broadened NIR pulse is separated from the XUV at the first perforated mirror (PM1). Subsequently, it is focused via a protected Ag mirror (M1) into a rotatable, free-standing 20-µm-thin BBO crystal ($\theta =44.3^{\circ }$, $\phi ={90}^{\circ }$) and NIR attenuated via a broadband, spectrally selective dielectric mirror (M2) before the DUV and XUV get recombined in a collinear fashion via the second perforated mirror (PM2) at the exit of the interferometer. The silicon wafer chicane (S1/S2) put at the NIR Brewster angle (75° angle of incidence) ensures both NIR reduction and high DUV throughput due to crossed polarizations of DUV and NIR, the latter being attenuated by more than four orders of magnitude. Another DUV reflecting, but NIR attenuating dielectric mirror in front of the second perforated mirror (PM2) is not shown. (b) Setup of the time-resolved photoelectron spectroscopy experiment for measuring electronic and nuclear dynamics in ozone: A highly pure ozone sample enters the interaction region below the electron time-of-flight detector (TOF) through a graphite coated glass nozzle perpendicular to the TOF axis orientation and the cross-polarized, collinear DUV-XUV-pulse train (along the $y$ axis). The DUV pulse is used to excite the ozone molecules from the ground $X$ state into the excited $B$ state, whereas the XUV continuously ionizes the ozone molecules at different delay times to keep track of the evolving molecular dynamics. Electronic excitation from $X\to B$ will only be accomplished if the DUV polarization points in the same direction as the molecule's dipole moment along the $y$ axis (red molecule) and will be thoroughly suppressed in all other cases of molecular orientation (blue molecule). The XUV released photoelectrons and their kinetic energy are measured with the TOF as a function of pulse delay τ between DUV and XUV. Here, τ > 0 refers to the situation where the DUV pump precedes the XUV probe pulse and vice versa.

Figure 4

Dissociation of ${\mathrm{O}}_3$ into diatomic oxygen, ${\mathrm{O}}_2$. Horizontal error bars estimate the maximum experimental inaccuracy for assigning a data point to a specific instant of time. Vertical error bars and red shading refer to 95% confidence interval of data and fit, respectively. Red line indicates fitted temporal evolution using a fit function as explained in the main text. (a) Right: The spectrogram, $S$($E,t$), consisting of all measured spectra exhibiting the temporal evolution of intramolecular dynamics of ozone is shown as a function of XUV-DUV time delay (negative delays: XUV precedes DUV), using a logarithmic color scale. Left: Typical ${\mathrm{O}}_3$ photoelectron spectrum and the energy domains relevant for data evaluation. These correlate with state-specific cation states (see main text). At zero delay the buildup of $B$ state-associated photoelectrons (uppermost box: 90–93 eV) is accompanied by a decrease of $X$ state-related photoelectrons (second and fourth boxes: 79.5–81 eV and 86.5–88 eV). After ∼20 fs a rise of photoelectrons emanating from diatomic oxygen, ${\mathrm{O}}_2$, shows up (third box: 82.5–85 eV), proving the successful dissociation of ozone. (b) Energy-integrated number of ground-state-related photoelectrons: Depletion of ground state is accomplished between −10 and 10 fs with a temporary increase around ∼25 fs most likely coming from ${\mathrm{O}}_2$-related photoelectron contamination. (c) Energy-integrated number of ${\mathrm{O}}_2$-related photoelectrons: Successful photolysis from ${\mathrm{O}}_3$ into ${\mathrm{O}}_2$ is accomplished for the first time between 10 and 20 fs, and a second time between 30 and 40 fs (first and second kink). For explanation see main text.

Figure 5

Temporal evolution of $B$-state-related photoelectron emissions and intramolecular dynamics obtained from MCTDH modeling. (a) Measured temporal evolution of $B$-state-related photoelectrons integrated over the energy domain relevant for cation states emerging from the excited $B$ state (90–93 eV) (for details regarding the dynamics see main text). (b) Simulated temporal evolution of $B$-state-associated photoelectrons (brown, blue, red, and black curves) integrated over the same energy range as in (a) and contrasted with the temporal evolution of the absolute value (purple curve) and the real part (light gray curve) of the electronic coherence between the ground $X$ state and the excited $B$ state. (c) Satisfactory qualitative agreement between (a) and (b) unveils the electronic and nuclear dynamics in ozone via comparison to MCTDH NWP propagation for five exemplary instants of time (I–V). Way before temporal overlap (−20 fs) there is no population on the $B$-state PES (I). At 0 fs, maximum excitation is achieved and located in close vicinity of the FC point (II). Around 20 fs, parts of the NWP have already dissociated, while another part of the NWP returns to the FC point for the first time (III). Minimum NWP density at the FC point is obtained around 34 fs (IV), before a second revival of NWP density is observed around 40 fs (V), being accompanied by repeated dissociation.