ABSTRACT
Acetylene (C2H2) is a simple hydrocarbon with a triple C-C bond and it is isoelectronic with N2 and CO with 14 electrons The ground-state electronic configuration is given by Mu-Tao et al (1990)
The length of the C-C bond is 0121 nmIn the ground state, the molecule is nonpolar, but acquires a dipole moment when certain modes of vibration are excited, as shown in Table 381The bond dissociation energy (H−CCH) is 578 eV, the ionization energy is 1140 eV, and the electron affinity is 049 eV
The molecule has five fundamental modes of vibration classified as shown in Table 381 (Kochem et al, 1985)
The ν4 mode is unobservable and the cross sections for other modes are presented in Table 385
See Table 382
381 Selected References for Data 259 382 Total Scattering Cross Section 260 383 Differential Scattering Cross Sections 260 384 Elastic Scattering and Momentum Transfer Cross Sections 261 385 Ro-Vibrational Scattering Cross Section 262 386 Ion Appearance Potentials 262 387 Ionization Cross Sections of Selected Hydrocarbons 262 388 Dissociative Ionization Cross Sections 262 389 Attachment Cross Sections 263 3810 Mobility and Drift Velocity 264 3811 Ionization Coefficients 266 References 267
TABLE 38.2 Selected References for Data on C2H2 Quantity Energy Range (eV) Reference
Cross Sections
Qi 10-600 Tian and Vidal (1998) Qi I0-800 Zheng and Srivastava (1996) QT 400-2600 Xing et al. (1995) Qdiff 5-100 Khakoo et al. (1993) Qel, Qinel 10-1000 Jain and Baluja (1992)
Qdiff (T) 10-200 Mu-Tao et al (1990)
QT 1-400 Sueoka and Mori (1989) Qa 0-12 Dressler and Allan (1987) QV 0-36 Kochem et al. (1985) Qa 0-15 Rutkowsky et al. (1980) Qdiff 100-1000 Fink et al. (1975) QM 001-006 Bowman and Gordon (1967) Qdiff 10-100 Hughes and McMillen (1933)
Swarm Properties We 06-10 Bowman and Gordon (1967)
α/N 125-9000 Heylen (1963)
Note: Bold font denotes experimental study Qa = Attachment; Qdiff = differential; Qel = elastic; Qi = ionization; Qinel = inelastic; QT = total; T = theory; We = electron drift velocity; E/N = reduced electric field; Td = Townsend
TABLE 38.1 Details of Vibration Modes of C2H2 Molecule
Designation Description εv
(eV) µ
(D) Representation
ν1 Symmetric C−H stretching 418 0 H−C-C−H ν2 Symmetric C≡C stretching 245 0 H−C-C−H ν3 Asymmetric stretching 409 007 H−C-C−−H ν4 Symmetric bending 73 0 ν5 Asymmetric bending 91 0243
Note:εv = vibrational energy (meV); µ = dipole moment (D)
See Table 383 The dominant features of the total scattering cross section are
1 Evidence of Ramsauer-Townsend minimum at 015 eV
2 A shape resonance at 26 eVC2H2 exhibits lowenergy shape resonance with designation 2Πg, common with N2 and CO The resonance is associated with predissociation of the molecule into C2H− and H fragments (Dressler and Allan, 1987)
3 A second peak at about 8 eV possibly due to dissociation and other inelastic collisions
4 A monotonic decrease for energy >10 eV
Peaks and valleys in the differential cross sections show the presence of interference effects between waves scattered from
different atoms of the molecule Figures 382 and 383 show the differential cross sections in the low-(≤100 eV) and highenergy ranges, respectively The highlights of differential
Total Scattering Cross SC2H2 Energy (eV) QT (10−20 m2) Energy (eV) QT (10−20 m2) Energy (eV) QT (10−20 m2) Energy (eV) QT (10−20 m2)
Dressler and Allan (1987) Sueoka and Mori (1989) Xing et al. (1994)
005 2410 10 181 12 209 400 538
010 2361 12 206 13 208 500 459
015 2327 14 229 14 201 600 406
025 2438 16 263 15 189 700 367
035 2538 18 294 16 189 800 319
045 2583 20 327 17 190 900 286
060 2630 22 340 18 182 1000 261
075 2671 25 358 19 186 1100 241
090 2706 28 344 20 177 1200 222
100 2736 31 326 22 166 1300 211
120 2795 34 296 25 159 1400 195
140 2884 37 283 30 149 1500 184
160 3026 40 269 35 146 1600 174
180 3185 45 255 40 133 1800 157
200 3362 50 237 50 125 2000 144
250 3572 55 238 60 120 2200 136
300 3529 60 241 70 113 2400 132
400 3273 65 246 80 108 2600 12
450 3190 70 242 100 93
500 3270 75 244 150 78
80 240 200 69
85 231 250 60
90 228 300 53
95 225 350 49
10 221 400 49
Note:The energy ranges are (1) 005 ≤ ε ≤ 5 eV (Dressler and Allan, 1987); (2) 1 ≤ ε ≤ 400 eV (Sueoka and Mori, 1989); (3) 400 ≤ ε ≤ 2600 eV (Xing et al, 1994)The relative cross sections of Dressler and Allan are normalized to the resonance peak measured by Sueoka and MoriSee Figure 381 for graphical presentation
cross sections as a function of energy and angle of scattering are (Khakoo et al, 1993)
1 The differential scattering cross section decreases mon otonically with increasing angle of scattering in the range of 10°–90° For angles >125°, there is increased backward scattering (Hughes and McMillen, 1933)
2 The differential scattering cross section shows a peak in the energy range from 5 to 10 eV
The measured integral elastic cross sections (Table 384) agree well as shown in the last column, except at 100 eV However, the theoretical values are much higher, particularly at energies below 100 eV and the differences have not been resolvedThe integrated and normalized values of elastic scattering cross section from Fink et al (1975) are taken from Karwasz et al (2001)
See Table 385
TABLE 38.4 Integral Elastic Scattering Cross Sections of C2H2
Energy (eV)
Qel
Hughes et al.
