Cross sections for ionization of tetrahydrofuran by protons at energies between 300 and 3000 keV

Double-differential cross sections for ionization of tetrahydrofuran by protons with energies from 300 to $3000\mathrm{keV}$ were measured at the Physikalisch-Technische Bundesanstalt ion accelerator facility. The electrons emitted at angles between ${15}^{\circ }$ and ${150}^{\circ }$ relative to the ion-beam direction were detected with an electrostatic hemispherical electron spectrometer. Single-differential and total ionization cross sections have been derived by integration. The experimental results are compared to the semiempirical Hansen-Kocbach-Stolterfoht model as well as to the recently reported method based on the dielectric formalism. The comparison to the latter showed good agreement with experimental data in a broad range of emission angles and energies of secondary electrons. The scaling property of ionization cross sections for tetrahydrofuran was also investigated. Compared to molecules of different size, the ionization cross sections of tetrahydrofuran were found to scale with the number of valence electrons at large impact parameters.

Figure 1

Variation of the magnetic field along the beam axis with (open symbols) and without (solid symbols) compensation. $Bx$ (red triangles), $Bz$ (green dots), and $By$ (blue squares) represent the orthogonal components of the magnetic field in the effusive gas jet direction, in the ion-beam direction, and in the direction perpendicular to the beam and gas jet, respectively.

Figure 2

$L_{2,3}{M}_{2,3}{M}_{2,3}$ Auger spectrum (symbols) of argon measured at ${90}^{\circ }$ by $1550\mathrm{keV}$ protons. The peaks are numbered arbitrarily for identification. The measured spectrum is fitted by the superposition (solid line) of Gaussians (dashed lines) with a FWHM of an estimated energy resolution of $1\mathrm{eV}$.

Figure 3

Experimental (symbols) and theoretical (solid line) differential cross sections of $100\mathrm{eV}$ electrons scattered elastically by argon. The theoretical values are obtained from the NIST data [40] convolved with a Gaussian with a width equal to the acceptance angle of $2^{\circ }$.

Figure 4

Uncorrected (blue dots) and corrected (black dots) experimental double-differential cross sections (DDCS) for ionization of THF by $2000\mathrm{keV}$ protons in comparison to the DDCS of adenine (red open triangles) and uracil (green open squares) reported by Itoh et al. [21, 22]. The reason and method used for the correction are explained in the text. The data are shown as a function of secondary electron energy for a sequence of emission angles from ${15}^{\circ }$ to ${135}^{\circ }$. The data sets of adenine and uracil were scaled with a factor $30/50$ and $30/42$, representing the ratio of the number of valence electrons.

Figure 5

Present experimental (symbols) double-differential cross sections (DDCS) for ionization of THF by protons in comparison to the theoretical DDCS obtained using the HKS model [45] (solid line) and the DRF model [25] (dashed line) for different proton energies: (a) 300, (b) 420, (c) 840, (d) 1200, (e) 1550, (f) 2000, and (g) $3000\mathrm{keV}$. The results are shown as a function of secondary electron energy for the emission angles from ${15}^{\circ }$ to ${75}^{\circ }$. For readability, the data of different emission angles are multiplied by the indicated factor.

Figure 6

The same as in Fig. 5, but for the emission angles from ${90}^{\circ }$ to ${150}^{\circ }$.

Figure 7

Present experimental (symbols) double-differential cross sections (DDCS) for ionization of THF by protons in comparison to the theoretical DDCS obtained using the HKS model [45] (solid line) and the DRF model [25] (dashed line) for different proton energies: (a) 300, (b) 420, (c) 840, (d) 1200, (e) 1550, (f) 2000, and (g) $3000\mathrm{keV}$. The results are shown as a function of emission angle for secondary electron energies $50\mathrm{eV}$ (black), $100\mathrm{eV}$ (red), $200\mathrm{eV}$ (green), and $500\mathrm{eV}$ (blue).

Figure 8

Present experimental (symbols) double-differential cross sections (DDCS) for ionization of THF by $1200\mathrm{keV}$ protons in comparison to the theoretical DDCS obtained using the DRF model [25] with (solid line) and without (dashed line) Salin's factor [50]. The results are shown as a function of emission angle for secondary electron energies $50\mathrm{eV}$ (black) and $500\mathrm{eV}$ (red).

Figure 9

Present experimental (symbols) double-differential cross sections (DDCS) for ionization of THF by protons with incident energies of (a) 300, (b) 1550, and (c) $2000\mathrm{keV}$. The results are compared to the DDCS of hydrogen [59], water [12], ethane and benzene as hydrocarbon [58], adenine [21], and uracil [22] for different proton energies: (a) $0.3$, (b) $1.5$, and (c) $2.0\mathrm{MeV}$. The data are shown as a function of emission angle for secondary electron energies $15\mathrm{eV}$ (black), $50\mathrm{eV}$ (red), $200\mathrm{eV}$ (green), and $500\mathrm{eV}$ (blue). The data sets of other molecules were multiplied by the ratio of the number of the valence electrons of THF to that of their valence electrons.

Figure 10

Present experimental (symbols) single-differential cross sections (SDCS) for ionization of THF by protons as a function of secondary electron energy. The data were obtained from the DDCS by numerical integration over the solid angle. The present results are compared to the theoretical SDCS calculated using the DRF model [24] (dashed line) for different proton energies. For better readability, the data for the different proton energies are multiplied by the indicated factor.

Figure 11

Present experimental (black dots) single-differential cross sections (SDCS) for ionization of THF by protons with incident energies of (a) 420 and (b) $2000\mathrm{keV}$. The data were obtained from the DDCS by numerical integration over the solid angle. The results are compared to the SDCS of adenine (red open triangles) and uracil (green open squares) reported by Itoh et al. [20, 21, 22] for different proton energies: (a) $0.5$ and (b) $2\mathrm{MeV}$. The data sets of adenine and uracil were multiplied by the ratio of the number of the valence electrons of THF to that of their valence electrons.

Figure 12

Present experimental (black dots) total ionization cross sections (TICS) of THF obtained by integrating the SDCS in comparison to the theoretical TICS of THF calculated using the DRF model [24] (blue dashed line) for different proton energies. The TICS of adenine (red open triangles) and uracil (green open squares) reported by Itoh et al. [20, 21, 22] are also shown for proton energies at $0.5$, 1, and $2\mathrm{MeV}$. The data sets of adenine and uracil were scaled by the ratio of the number of valence electrons.

Figure 13

The experimental total ionization cross sections (TICS) of various molecules for $2\mathrm{MeV}$ proton impact as a function of the number of valence electrons. The data are taken from [1] for molecular hydrogen (${\mathrm{H}}_2$), from [13] for water (${\mathrm{H}}_2\mathrm{O}$), from [60] for monomethylamine (${\mathrm{CH}}_5\mathrm{N}$) and dimenthylamine (${\mathrm{C}}_2{\mathrm{H}}_7\mathrm{N}$), from [22] for uracil (${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2\mathrm{O}2$), and from [21] for adenine (${\mathrm{C}}_5{\mathrm{H}}_5{\mathrm{N}}_5$). The TICS of THF is also plotted.