Photodissociation Experiment of Phenol at 213 nm

The H atom TOF spectra are illustrated in Figure 3-3 for two pump laser positions, at the crossing point (0 mm) and at the second position (5.5 mm upstream of molecular beam). Two components in the TOF spectrum of 0 mm can be observed: a sharp and fast component (component I) at arrival time 5-10 us and a broad and slow component (component II) located from 10 us to 30 us. When the pump laser was moved 5.5 mm upstream of the molecular beam, the shape of the TOF spectra changed. The components I and II in the TOF spectrum become small and the third component (component III) located in the region from 20 μs to 100 μs can be clearly observed.

Figure 3-3. H atom TOF spectra at 213 nm. Black line represents the spectrum at crossing point.

Red line represents the spectrum at 5.5 mm upstream of molecular beam. The intensity of different positions are scaled with different factor for comparison

Component III is not obvious in the TOF spectrum of 0 mm because of the large intensities of components I and II. Component III is buried in component II and it becomes clear when the intensity of component II decreases. Because the acceptance angle of the detector, the fragments forming in the first 2.5 μs were detected when pump laser was located at 0 mm, while the fragments forming in the 0.5-6.5 μs were detected at 5.5 mm. The relative intensity changes of these three components indicate that the components I and II after subtracting component III are produced within 0.5 μs and the generation of the component III can last to longer than 0.5 us. We assigned components I and II to the dissociation in the electronic excited state and component III to the dissociation of highly vibrationally excited phenol in the electronic ground state.

The corresponding translational energy distributions are illustrated in Figure 3-4.

The translational energy distribution obtained from the pump laser located at 0 mm shows two components. The fastest component, corresponding to the component I in TOF spectra, centers at ~12000 cm-1 and reaches the maximum available energy. We assigned the fastest component as dissociation in the electronic excited state forming electronic ground state phenoxyl radical product. The slow component with translational energy <8000 cm-1, corresponding to the component II in TOF spectra, peaks at 2000 cm-1. Two dissociation channels contribute to this slow component:

dissociation in the electronic excited state forming phenoxyl radical in the electronic

excited A state, or dissociation in the ground state. The translational energy obtained from the pump laser beam located at 5.5 mm (component III) mainly distributed below 3000 cm-1. It is solely contributed from the dissociation in the ground state. The translation energy distributions of dissociation in the excited state producing phenoxyl radicals in A state can be obtained by subtracting the component III from component II with proper factor as described in chapter 2.

Figure 3-4. Translational energy distribution of H atom elimination channel at 213 nm. Vertical arrows indicate the maximum available energy of one photon dissociation to the phenoxyl radical ground state, A state, and B state respectively. Black line represents the spectrum at crossing point.

Red line represents the spectrum at 5.5 mm upstream of molecular beam. The intensity of different positions are scaled with different factor for comparison

The signal intensity change as the function of the pump laser position for fast component (>10000 cm^-1) and slow component (<3000 cm^-1) are illustrated in the Figure 3-3. As described in chapter 2, the intensity change as the function of the pump laser position for fast component can be regarded as an instrument response function D(P) related to the distribution of pump laser beam, the detector acceptance region x1

and x2. D(p) become zero when p < -5 and thus the slow component at position p<-5 is mainly from component III. We fit the intensity of slow component as the function of pump laser position when p< -5 with the third term of Equation (2-15) to get the relative branching ratio and lifetime of ground state dissociation channel as illustrated in Figure 3-5. The fitted lifetime is 1.5±0.3 μs.

Figure 3-5. Intensity change as the function of the pump laser position at 213 nm for fast component (black square) and slow component (red dot) respectively

Figure 3-6. Fitting of Relative intensity change of slow component as the function of position at 213 nm. Solid square represents the experimental value. Red line represents the fitting curve. The fitting lifetime is 1.5±0.3 μs.

From Figure 3-6, we can find the contribution of component III at the crossing point and subtract it from the spectrum to get the translational energy distribution of excited state dissociation and deconvolute it with two components, i.e. dissociation in the excited state forming phenoxyl radical X and A state, as shown in Figure 3-7.

Finally we can get the relatively branching ratio of OH bond fission of different dissociation channel as 0.07, 0.60, 0.32 for ground state dissociation and excited state dissociation forming phenoxyl radical X and A state respectively.

Figure 3-7. Translational energy distribution of excited state dissociation channel at 213 nm.

Vertical arrows indicate the maximum available energy of one photon dissociation to the phenoxyl radical ground state, A state, and B state respectively. Black line is the experimental data. Blue l and red line are the deconvolution results of black line.

Anisotropic parameter β₂ is also calculated by measuring the signal change with

the angle of laser polarization and fit with the equation as shown in Figure 3-8. The fitting β2 for the fast component is 0.1±0.05, a small but non-zero value. This is an

indication that lifetime of fast component is less than 100 ps, which is in consistence with the assignment of dissociation in the repulsive state.

Figure 3-8. Signal change with the angle of laser polarization at 213 nm. Black line represents the laser polarization parallel to the detection axis. Red line represents the laser polarization perpendicular to the detection axis.