ABSTRACT
Hydrogen fluoride (HF) is a polar, electron-attaching molecule with 10 electronsIt has electronic polarizability of 089 × 10−40 F m2, dipole moment of 1826 D, and ionization potential of 1604 eVThe energy for the fundamental vibrational mode is 5131 meV
Table 211 shows the selected references for data on HF
The rotational constant for HF molecule is very low, 26 meV, and the energy for the first rotational excitation is 52 meV Knoth et al(1989) have measured the differential scattering cross sections for the first vibrational level, v = 0 → 1, simultaneous with rotational excitation as shown in Table 212
HF, like HCl, shows a strong threshold peak of 10−19 m2 in vibrational excitation cross section, with scattering becoming isotropic (Allan and Wong, 1981)
The appearance potential of F− ion is given by the following equation (Forst and McDowell, 1958):
AP(F−) = DE(HF) − EA(F) + KE (211)
where AP is the appearance potential of F− ion DE(HF) is the dissociation energy of the HF molecule EA(F) is the electron affinity of the F atom KE is the kinetic energy of the products
211 Selected References for Data 139 212 Ro-Vibrational Excitation Cross Section 139 213 Attachment Processes 139 214 Attachment Rate Constant 140 References 140
TABLE 21.2 Differential Cross Sections for the First Vibrational Level for HF
Differential Cross Section (10−20 m2/sr)
Energy (eV)
Angle (°) 0.63 0.9 1.2
15 040
30 047 012 0067
45 054 0083 0045
60 060 0078 0043
75 061 0086 0044
90 061 010 0048
105 060 010 0054
120 058 011 0060
135 060 012 0064
Source: Adapted from Knoth, G et al, J. Phys. B: At. Mol. Opt. Phys, 22, 2797, 1989
TABLE 21.1 Selected References for Data
Parameter Range: eV, (K) Reference
Qv 05-10 Knoth et al. (1989) Qel 05-10 Rädle et al. (1989) ka (300, 510) Adams et al. (1986) Qa 0-4 Allan and Wong (1981) Qv 0-6 Rohr and Lindner (1976)
Note:Qa = attachment cross section; Qel = elastic scattering cross section; Qv = vibrational excitation cross section Bold face indicates experimental study
The dissociation energy of the hydrogen iodide molecule is smaller than the electron affinity of the iodine atom, rendering the electron affinity negative, according to Equation 211 The potential energy diagrams for the molecules in the first and last row of Table 213 are shown in Figures 211 and 212
Adams et al (1986) have measured the attachment rate constant for HF, using flowing afterglow/Langmuir probe technique, as <1 × 10−17 m3/s
TABLE 21.3 Attachment Processes in Hydrogen Halides
Experimental
Ion DE, eV
(Molecule) EA (Atom) AP, Equation
21.1 Onset (eV)
Peak (eV)
F− 583 (HF) 363 (F) 220 188 40
Cl− 443 (HCl) 378 (Cl) 065 062 077
Br− 375 (HBr) 354 (Br) 021 010 021
I− 306 (HI) 324 (I)a −018 003 005
Source:Adapted from Frost, DCand CAMcDowell, J. Chem. Phys., 29, 503, 1958
a More recent determination of electron affinity of I is 3059 eV (Klar et al, 2001)
En er
gy (e
V)
H + F
H + F-
D(HF) EA (F-)
1 Nuclear separation
En er
gy (e
V)
2 Nuclear separation
r0 3
HI
H + I-
H + I
D(HI) E(I)
