Valence photoelectron imaging of molecular oxybenzone

Abstract

An oxybenzone molecule in the gas phase was characterized by mass spectrometry and angle-resolved photoelectron spectroscopy, using both single and multiphoton ionization schemes. A tabletop high harmonic generation source with a monochromator was used for single-photon ionization of oxybenzone with photon energies of up to 35.7 eV. From this, vertical ionization and appearance energies, as well as energy-dependent anisotropy parameters were retrieved and compared with the results from DFT calculations. For two-photon ionization using 4.7 eV light, we found a higher appearance energy than in the extreme ultraviolet (EUV) case, highlighting the possible influence of an intermediate state on the photoionization process. We found no differences in the mass spectra when ionizing oxybenzone by single-photons between 17.2 and 35.7 eV. However, for the multiphoton ionization, the fragmentation process was found to be sensitive to the photoionization order and laser intensity. The "softest" method was found to be two-photon ionization using 4.7 eV light, which led to no measurable fragmentation up to an intensity of 5 × 10¹² W cm^-2.

Fig. 1. (A) Structure of OB in its enol form with an indication (blue square) of the suggested structure for the m/z = 151 amu fragment. (B) MS (normalized to the peak centred at 227 amu) collected upon ionization with a series of photon energies in the EUV range. Insets are zoomed to the mass fragments of OB seen in the MS with the noise level indicated in gray. The helium peak is also indicated in the main MS. The mass accuracy of the fragments is estimated to be ±1 amu.

Fig. 2. MPI-MS spectra recorded with: (A) 266 nm with an intensity of 5 × 10¹² W cm⁻²; (B) 400 nm with a relatively low intensity of I ∼ 6 × 10¹² W cm⁻² (C) 400 nm with a relatively high intensity of I ∼ 2 × 10¹³ W cm⁻². The masses of the observed fragments are indicated in the figure with an accuracy of ±1 amu.

Fig. 3. (A) and (B) examples of two photoelectron images taken at different photon energies and their corresponding reconstructions (as indicated). E⃑ indicates the polarization direction of light in the experiment; (C) EUV PES and calculated spectrum (using method (i) (indicated as “DFT”) convoluted with Gaussian of FWHM = 0.71 eV). Bands (as assigned in Table 1) are marked in gray with their corresponding band index. Data from the center of the image (KE < 0.1 eV) has been excluded from these traces.

Fig. 4. Comparison of PES taken with VIS (400 nm), UV (266 nm) and EUV (20.4 eV) photon energies (PES present data from KE > 0.1 eV). Dotted black lines in the inset are linear fits to the onset of the 20.4 eV and 266 nm signals and the noise level. Dashed magenta spectrum as calculated by method (i) and convoluted with a Gaussian (FWHM = 0.71 eV).

Fig. 5. Visualization of the 7 energetically highest lying occupied orbitals of neutral oxybenzone as obtained by method (i). The 5 energetically highest lying occupied orbitals contribute to the first VBE band (band 0) and exhibit predominantly π-character while orbitals lying lower in energy (separated by dashed line) exhibit mostly s-character.

Fig. 6. Evolution of the β-parameter as a function of electron kinetic energy for the first 3 valence bands. The shaded area indicates the uncertainty.