Photofragmentation of tetrahydrofuran molecules in the vacuum-ultraviolet region via superexcited states studied by fluorescence spectroscopy

Photofragmentation of tetrahydrofuran molecules in the vacuum-ultraviolet region, producing excited atomic and molecular fragments, has been studied over the energy range 14–68 eV using photon-induced fluorescence spectroscopy. Excited hydrogen atoms H($n$), $n$ $=$ 3–11, have been detected by observation of the ${\mathrm{H}}_{\alpha }$ to ${\mathrm{H}}_{ı}$ lines of the Balmer series. The diatomic CH($A^2\Delta$), CH($B^2{\Sigma }^{-}$) and ${\mathrm{C}}_2(d^3{\Pi }_g)$ fragments, which are excited to low vibrational and high rotational levels are identified by their $A^2\Delta \to X^2{\Pi }_r$, $B^2{\Sigma }^{-}\to X^2{\Pi }_r$ and $d^3{\Pi }_g\to a^3{\Pi }_u$ emission bands, respectively. Dissociation efficiency curves for CH$(A^2\Delta )$ and H($n$), $n$ $=$ 3–7, have been obtained in the photon energy ranges from their appearance thresholds up to 68 eV. The appearance energies for CH$(A^2\Delta )$ and H($n$), $n$ $=$ 3–7, have been determined and are compared with estimated fragmentation energy limits in order to discuss the possible fragmentation processes. In the present studies, superexcited states of tetrahydrofuran are found, which dissociate into the above excited atomic and molecular fragments.

Figure 1

Schematic diagram of the THF molecule, ${\mathrm{C}}_4$H${}_8$O, in the ${\mathrm{C}}_2$ (twisted) conformation showing labeling of the atoms. Carbon atom, gray; oxygen atom, red; and hydrogen atom, dark blue.

Figure 2

Schematic view of the experimental system.

Figure 3

Relative detection efficiency of the experimental system determined in the 300–800 nm wavelength range.

Figure 4

Fluorescence emission spectrum measured for THF at a photon energy of 62 eV. The spectrum was corrected for the wavelength dependence of the sensitivity of the detection system.

Figure 5

Fluorescence emission spectrum of THF measured in the 410–445 nm wavelength range at a photon energy of 62 eV. The positions of the rotational lines of the $Q$, $P$, and $R$ branches of the CH$(A^2\Delta \to X^2{\Pi }_r)$ bands are indicated by the vertical bars.

Figure 6

Fluorescence emission spectrum of THF measured in the 380–410 nm wavelength range at a photon energy of 62 eV. The positions of the rotational lines of the $Q$, $P$, and $R$ branches of the CH$(B^2{\Sigma }^{-}\to X^2{\Pi }_r)$ bands are indicated by the vertical bars.

Figure 7

Fluorescence emission spectrum of THF measured in the 460–515 nm wavelength range at a photon incident energy of 62 eV. The positions of the vibrational lines of the ${\mathrm{C}}_2(d^3{\Pi }_g\to a^3{\Pi }_u)$ bands are indicated by the vertical bars.

Figure 8

The emission intensity of the lines of the Balmer series as a function of the principal quantum number $n$. The solid line shows the best fit to the experimental points, which was obtained for an $n^{-3.74}$ dependence.

Figure 9

Dissociation efficiency curves for production of the H($n$), $n$ $=$ 3–7, atoms measured in the 17–20 eV photon energy range. The (red) arrows indicate appearance energies of the fragments.

Figure 10

Dissociation efficiency curves for production of the H($n$), n $=$ 3–7, atoms measured in the 18–65 eV photon energy range. The bands fitted to the experimental curves are shown by the dashed lines and the final fits are shown by the solid lines.

Figure 11

Dissociation efficiency curve for production of the CH $(A^2\Delta )$ fragments measured in the 14.6–20 eV photon energy range. The bands fitted to the experimental curves are shown by the dashed lines and the final fit is shown by the solid line. The vertical bars show the positions of the photoionization 6b/5a″, 7a/8a′, and 5b/7a′ bands.

Figure 12

Dissociation efficiency curve for production of CH $(A^2\Delta )$ fragments measured in the 20–60 eV photon energy range. The bands fitted to the experimental curves are shown by the dashed lines and the final fit is shown by the solid line.