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Collisional ionization of highly excited neon atoms by nonpolar hydrocarbons
Chemical Physics 64 (1982) 381-387 North-Holland Publishing Company
COLLISIONAL BY NONPOLAR
IONIZATION
OF HIGHLY
Masashi UEMATSU, Hisato HIRAISHI, and Kozo KUCHITSU Deparfment of Chemistry, Faculty of Science, Hongo, Bunkyo-ku, Tokyo 113, Japan
Received
6 July
EXCITED
NEON
ATOMS
HYDROCARBONS
The
Tsutomu University
FUKUYAMA*,
Tamotsu
KONDOW
of Tokyo,
1981
When benzene, acetylene and ethylene were allowed to collide with Ne Rydberg atoms (17 d n s 40), significant Ne+ ions were observed. Their cross sections were estimated, with reference to the ionization by HzO, to be as large as 10~‘4-10~‘s cm’. However, no Nef signals were observed when ethane, methane and N2 were used as targets. Theoretical estimates of the cross sections for ionization by interaction of the quadrupole moment and polarizability of the first three molecules are several orders of magnitude smaller than the experimental cross sections given above. A by Christophorou et al., similarity of this phenomenon to the scattering of a thermal electron by benzene etc., observed is suggested.
1. Introduction
Recent experimental [l, 21 and theoretical [3] studies have shown that collisional ionization of a highly excited Rydberg atom by a polar molecule can be understood as scattering of the Rydberg electron in the highly excited atom by the target molecule [4]. The Rydberg electron causes rotational deexcitation of the molecule by interaction with the target dipole, and it is ejected from the atom by taking up the released rotational energy. Even if the target is nonpolar, a Rydberg atom can still be ionized if the target molecule has a positive electron affinity, because the Rydberg electron can be captured by the target [4-71. Sulfur hexafluoride is the best known example. To the contrary, there seems to have been no experimental evidence for the ionization of a Rydberg atom by a nonpolar target having a
negative electron affinityt. Hotop and Niehaus [8] attempted to detect the Ar+ ions resulting from the argon Rydberg atom colliding with a number of “nonpolar” molecules (Hz, NZ, O2 and CH& but they could not observe any noticeable ion signals. A theoretical explanation was given by Matsuzawa [3]. He calculated the cross section for ionization of a Rydberg atom by interaction with the quadrupole moment of a “nonpolar” molecule, such as Nz, and he estimated the cross section to be several orders of magnitude smaller than that caused by the dipole interaction of a polar target such as H20. In spite of these circumstances, it seemed questionable to conclude that all the “nonpolar” molecules were very poor ionizers of a Rydberg atom. In our attempt to find a suitable target, a guiding principle was provided by the similarity of a Rydberg electron to a free electron with thermal energy [3,4]. t A great
* Present Studies,
address: Ibaraki
National Institute 305, Japan.
0301-0104/82/0000-0000/$02.75
for
Environmental
study
@ 1982 North-Holland
majority
of the nonpolar
molecules,
negative electron affinities, are represented as “nonpolar”
molecules.
which have in the present
382
M.
Uematsu
et al. / Collisional
ionization
According to the electron swarm experiments of Christophorou et al. [9] for a number of gas molecules containing r-electrons, they conclude that the mobility of thermal electrons decreases, i.e. the scattering cross section of thermal electrons increases, as the number of the doublyoccupied r-orbitals in the target molecule increases. For instance, benzene was estimated to have a sizable scattering cross section of thermal electrons [9]. Therefore, benzene was first chosen as a test target in our study. A preliminary experiment indicated that benzene could indeed ionize a Rydberg atom [lo]. A subsequent refinement of the apparatus and experimental procedures has resulted in the present report. Collisional ionization of neon Rydberg atom, Ne**, with benzene and a few other target molecules, M,
(1)
Ne**+M+Ne’+M+e,
was observed. Neon was chosen because it was then possible to use its metastable (3P0,2) atoms, Ne”, for checking the effect of polar impurities by Penning ionization, Ne*+M+Ne+M++e.
of highly
excited
neon
atoms
chamber, which had an open bottom. This gauge was calibrated against an MKS pressure gauge (220B). The Ne+ signal intensity was found to be proportional to the target pressure. The relative cross sections were estimated from the slope of this linear relation. 2.2. Ionization
of Ne ** by nonpolar molecules
Significant Ne’ signals were observed when benzene, acetylene and ethylene were used as targets. The estimated upper and lower bounds of the cross sections for ionization, normalized to the ionization cross section for H20, are listed in table 1. On the other hand, when ethane, methane and nitrogen were used as targets, no Ne+ signal exceeding the noise level was observed. The main precursor of the observed Ne’ ions was proved to be Ne** by the following
Table 1 Observed
(2)
and calculated
cross
Observed d, relative d)
sections
a)
Calculated
‘)
2. Experimental
and results Hz0
2.1. Apparatus
C&s C2H2
The details of the apparatus and the experimental procedure have been described in our previous paper [ll]. Neon gas was introduced into the excitation region and was bombarded by an electron beam (50 eV). Charged particles were deflected from the neon beam by’an electrostatic field of about 200 V/cm, which corresponds to the ionization threshold for n -40; neutral species containing Rydberg atoms and metastable atoms, Ne*(3P2,0), were thereby allowed to enter the collision chamber. The pressure of the target gas in the collision region was typically 1 x 10e4 Torr. The ions formed in the collision were focused by ion lenses and mass-analyzed by a quadrupole mass filter. The target pressure was measured by an ionization gauge installed beneath the collision
C2H4 GH6 CH4
N2
1 2x10-z-4x10-3 1 x 1o-2 - 3 x 10m3 5x10-4-2x10-4 not observed e, not observed ” not observed ‘)
3.6 x 10-l’ 1.3xlo-‘s 3.6x lo-l6 5.4x10-17h) 4.1 x lo-l8 0
2.2 x lo-‘6
d)
relative
Cm2
f*g)
g.9
1
3.6 1.0 1.5 1.3 0 6.1
x x x x
lo-’ 1o-4 1o-5 1o-6
x lo-’
a) Cross sections for ionization of Ne** Rydberg atom. s) Estimated upper and lower bounds are listed. ‘) Ionization by quadrupole interaction estimated by use of the quadrupole moments [14] and the rotational constants [15]; see text. d, Normalized to the cross section for HzO. e, Smaller than 1 x 10m4. ” Ionization by dipole interaction, calculated by use of the dipole moment, 0.727 au [16]. ” These values are consistent with those estimated in ref. [3]; note, however, that no direct comparison can be made, because the n-distributions assumed in ref. [3] and in the present study are different. h, Crude estimate based on the symmetric-top approximation. i, Based on the rotational constant taken from ref. [17].
M. Uematsu
et al. / Collisional
ionization
measurements. Benzene was used as a target, since benzene gave the highest Ne+ intensity among the target molecules studied. (a) Excitation function: The Ne+ intensity was measured by varying the bombarding electron energy from 10 to 150 eV. The electron energy scale was calibrated with reference to the appearance potential of the Ne*(3P2,0) atoms, 16.6 eV, as measured by the &Hi ions formed by Penning ionization (2). The observed dependence of the Ne’ intensity on the electron impact energy is shown in fig. la. The onset of the Net signal is found to be 21.4kO.3 eV, which is nearly equal to the ionization potential of Ne, 21.56 eV. In addition, the curve shown in fig. la has a shape nearly identical with the excitation function of Ne** (fig. lb) determined from the collisional ionization (l), where M = HZO. (b) Field ionization: A field ionizer, composed of a pair of parallel-plate electrodes [ 111 with a spacing of 4 mm, was placed between the excitation region and the collision chamber. The Ne+ intensity was measured as a function of the electrostatic voltage applied to the electrodes. When the field strength was varied from 0 to 7.5 kV/cm, a uniform decrease in the Ne’ intensity was observed (see fig. 2). This dependence was ascribed to the loss of Ne** from the gas beam by field ionization. This measurement was used to make a crude estimate of the distribution of the principal
of highly
excited
neon
atoms
383
quantum number, n, of Ne** in the collision region [12]. On the basis of the calculation by Bailey et al. [13] on the critical field strength for ionization of H** atoms, the n distribution was estimated to range from about 17 to 40 with a peak at about 25. 2.3. Check of impurity
The benzene sample was of spectral grade (Nakarai Chemicals Ltd.) with quoted purity of 99.93%. Since a polar molecule ionizes Ne** much more efficiently than benzene, the presence of polar impurities was examined. The only impurity found in a gas chromatogram was toluene, the concentration being estimated to be 0.06%. The amount of thiophene was estimated to be 0.2 ppm by a calorimetric analysis. In order to remove water arising from ambient humidity, molecular sieve was placed in the sample holder, and the inlet copper tube which led the sample gas into the collision chamber was evacuated at about 150°C before the experiment. The concentrations of polar impurities were further examined by observation of impurity ions with the aid of a quadrupole mass spectrometer. When the benzene sample was introduced into the collision chamber, ions with masses 18 (H20’) and 92 (&H&H:) were observed in addition to Nef and several ions originating from C6H6, but no ions ascribable to
.
*. a**
a+
H20-N;+ .
(b)
.
.
.
Hz0 + c
.
0
0 .
l
,
,
1
,
50
Electron
,
,
,
1
,
100 Energy/eV
Fig. 1. The intensity of Ne+ produced in the collision function of the electron energy bombarding Ne.
.
.
.
.
I
1.
50
150
Electron of Ne**
with
(a) C6H6
#
.
*
I
.
100 Energy/eV
and (b) HZO,
plotted
.
.
.
1
150
in arbitrary
units
as
a
M.
384
2
Uematsu
et al. / Collisional
tc-+c&’ Ne’+ %“6+
=
ionization
of highly
excited
neon atoms
trap distillation. Water impurity was examined by the method described above. For acetylene (Matheson, purity 99.8%, the remaining substances being oxygen and nitrogen molecules) no impurity was observed in the mass spectrum.
3. Discussion
Fig. 2. The intensity of Net produced in the collision of Ne** with C6H6, plotted in arbitrary units as a function of the electrostatic field strength. The principal quantum numbers, n, which correspond to their critical field strengths, are estimated approximately [ 121 and indicated on the abscissa.
thiophene or any other impurity were observed. The H20f and C7Hi ions were produced by Penning ionization of the impurities [process (2), where M = HZ0 or ChHsCH3]. These impurities should also produce Ne’ ions by collisional ionization (l), which takes place simultaneously with the Penning process (2). The contribution of these polar impurities to the Ne+ signal were estimated in the following way. In a typical run, the total Net signal was 2000 cpm, while the HzOC signal was 30 cpm. When pure water was introduced into the collision chamber, the intensity ratio of the Ne’ and HzOC ions produced by processes (1) and (2), respectively, was 3.3. Therefore, the intensity of Ne+ due to the water impurity in the benzene sample was estimated to be about 100 cpm, namely 5% of the total Ne’ signal. Similarly, the contributions from toluene and thiophene were estimated to be 0.2% and less than 0.2%, respectively. Thus the polar impurities can account for only about 5% of the total Ne’ intensity observed; the remaining part of the Ne’ intensity is interpreted as originating from the collisional ionization by benzene. All the other gases used as targets, except for methane and nitrogen, were purified by trap to
The present study shows that three nonpolar molecules, benzene, acetylene and ethylene, have appreciable cross sections for ionization of neon Rydberg atoms. The cross sections for benzene and acetylene are estimated to be as large as 10-‘4-10-‘5 cm2 and are only two or three orders of magnitude smaller than that by a typical polar molecule, water. On the other hand, the cross sections for ionization by ethane, methane and nitrogen are all too small to be measurable, possibly less than 1 x lo-i6 cm*. The origin of the interaction causing ionization is yet uncertain; none of the following three mechanisms seems to contribute sufficiently to the cross sections. (1) Quadrupole interaction: According to Matsuzawa’s theory, the cross section for ionization by N2 due to quadrupole interaction is several orders of magnitude smaller than that by HZ0 [3]. In order to examine whether the same rule applies to the other targets, the cross sections for ionization by interaction of a quadrupole moment of the target were calculated. His formula for a linear molecule was used for NZ and C2H2. An analogous formula for a symmetric-top molecule was derived in the present study (see appendix) and applied to C6H6 and C2H6. The same equation was also applied to C2H4, which was approximated as a symmetrictop molecule with the rotational constants A and (B + C)/2; the quadrupole moment, Q,, was assumed to be the zt component of the quadrupole tensor. Because of these simplifications, the cross section for CZH4 is only an order-of-magnitude estimate. The molecular constants used in the present calculations were taken from refs. [14-171. The cross sections thus estimated were normalized to that by water calculated by the use of Matsuzawa’s
M.
Uematsu
et al. / Collisional
formula [3]. As shown in table 1, the calculated relative cross sections for ChH6, &Hz and C2H4 are several orders of magnitude smaller than the corresponding experimental estimates. Thus it is concluded that the quadrupole interaction is not a major mechanism of the collisional ionization by any of these molecules. (2) Polarization: Rydberg atoms can also be ionized by an induced dipole moment of the target. When this polarization effect is taken into account, the cross section for a symmetrictop molecule is given by the cu’-dependent terms in eq. (A.7). An order-of-magnitude estimation of these terms gives a cross section which is at most comparable with the contribution from the quadrupole interaction. A similar conclusion is reached for linear molecules. Hence, the effect of polarization of the target cannot remove the large discrepancy between the observed and calculated cross section by the three molecules. (3) Temporary negative-ion formation: If the Rydberg electron attaches to the target molecule forming a temporary negative ion and rapidly undergoes autodetachment, then the collisional ionization may have a large cross section. However, this possibility can be ruled out because the vertical electron affinities reported for benzene, acetylene and ethylene are -1.14 [18], -2.7 [19] or -2.6 [20], and -1.76 [18] eV, respectively, whereas the kinetic energy of the Rydberg electron is only about 10 meV, being insufficient for such an attachment process. 4. Concluding +electrons
remarks: Interaction
involving
ionization
of highly
excited
neon atoms
38.5
reported to be much larger than those by methane, nitrogen and ethylene [21]. Our present observation seems to support the prediction that “nonpolar” polyatomic molecules containing r-electrons should be good scatterers of a Rydberg electron as well as a thermal electron. Since nitrogen has a negligible cross section, either the molecular size or the vibrational degrees of freedom, or both, seem to contribute to the cross section. A further study is being undertaken for a more quantitative correlation of the cross sections for ionization with the mobilities of thermal electrons and for a better understanding of the mechanisms of these processes.
Acknowledgement
The authors are indebted to Drs. Y. Itikawa, M. Matsuzawa and I. Shimamura for their helpful suggestions, particularly in regard to the calculations given in the appendix. The present study has been supported by a grant from the Mitsubishi Foundation.
Appendix. Cross section for ionization of a Rydberg atom by the quadrupole moment of a symmetric-top molecule
The cross section for collisional ionization, A**(n)+B(JK)+A++B(J’K’)+e(&),
64.1)
has been given by Matsuzawa [3] as (T(n, JK + E, J’K’) k+k’
The observed cross sections for benzene and acetylene are two orders of magnitude larger than those for ethane and methane. This indicates that the ionization of a Ryberg atom takes place much more easily in the presence of r-electrons in unsaturated hydrocarbon targets. As mentioned in section 1, the similarity of the scattering of a Rydberg electron to that of a thermal electron is suggestive. The scattering cross section of a thermal electron by benzene is
2q.L2 =2 k
dFn(Q, E) I
de
Ik-k’l
x 1fe(JK -+J’K’))‘C? dQ, in terms of the following J, K: rotational quantum B, E: energy of the ejected CL: reduced mass of the
64.2)
symbols: numbers of molecule electron, colliding system,
M
386
Uematsu
et al. / Collisional
k and k’: wave numbers of the relative motion
before and after the collision, dF,(Q, E)/d& : form factor density per unit range of the ejected electron energy at momentum transfer Q; the binary-encounter theory gives eq. (14) of ref. [3],
of highly
excited
neon
atoms
described in ref. [3], it is shown that a(n ; JK + J’K’) 2K2(J2 -K2) -p, II2 15 0E [ J(J + l)(J - 1)(2J + 1)
=-4&r
(J2 -K2){(J - 1)’ -K2} +J(J-1)(2J+1)(2J-1)
dF,,(Q, E)/dE = (29/3&) x Q’/{(Q’-2~
ionization
- l/r~~)~+4Q~/n~}~,
fe(JK +J’K’): scattering amplitude tron scattering process,
(A.3)
for the elec-
1
~o)+&~Qp~'Lqh so)
x [Q;J,,(n,
41,
+&~2~‘2~4,h
(A.7)
where e +B(JK) + e’+ B(J’K’).
These formulas have been used for calculation of the ionization cross sections for a polar symmetric-top molecule and for a nonpolar linear molecule with a quadrupole moment (see eqs. (lob) and (12) of ref. [3]). In the present study, the above theory has been extended to the case of a nonpolar symmetric-top molecule with a quadrupole moment Q,. For this purpose, use is made of eq. (2.16) of ref. [22], which leads to lfel* = (klk’h,(JK 2J’+l
ZZ-
?r 00
J (K
+J’K’) J’ -K’
1 2 -m > 2
X
dr r2 utm(r)
II
idQr) ,
(A.3
0
where qf,,, is the differential cross section expressed in terms of the 3-j symbol, UL,,,is the asymptotic form of the potential, and il(Qr) is the spherical Bessel function. Becausesthe interaction is caused by a quadrupole moment, 2 is taken as 2. Then the selection rule for deexcitation is AJ = -1 and -2 [(2.7a) of ref. [22]]. The m = 0 case can be treated as follows. When the polarization interaction is taken into account, the potential is given, following eq. (3.1) of ref. [23], as zIzo(r) = -(47r/5)“2Qpr-3
cc
(A.4)
- (r/5)1’2a’r-4,
(A.6)
where (Y’ is the anisotropy of the polarizability of the molecule. Following the procedure
Ip,(n,4 = I [dFn(Q,41dElQp dQ,
64.8)
0
E is the energy of the relative motion, and e. is the difference between the transition energy and the binding energy. The first term in the second brackets represents the quadrupole interaction, while the second and third terms are due to the polarization interaction. The n-dependence of the cross section is obtained by taking the average over the thermal rotational distribution. The cross sections calculated in this way are further averaged over the experimental n-distribution function, estimated as described in section 2.2. The cross sections thus obtained are listed in table 1. Since benzene has a six-fold axis and ethane has a three-fold axis of symmetry, their quadrupole moments are given by independent scalars [14]. Therefore, the m=O case is sufficient for calculation of the cross sections. The above eq. (A.7) is also applicable to a nearly symmetric-top molecule such as ethylene. However, since ethylene has two independent components representing the quadrupole moment tensor [14], the case for m # 0 should also be considered. A numerical calculation for m # 0 is hampered by the lack of reliable experimental or theoretical values of these tensor components in addition to the complexity of the equation. Nevertheless, an estimate based on the m = 0 case, listed in table 1, seems to give a reasonable order of magnitude of the cross sections.
M.
Uematsu
et al. / Collisional
References [l]
M. Matsuzawa and W.A. Chupka, Chem. Phys. Letters 50 (1977) 373. [2] F.G. Kellert, K.A. Smith, R.D. Rundel, F.B. Dunning and R.F. Stebbings, J. Chem. Phys. 72 (1980) 3179. [3] M. Matsuzawa, J. Electron Spectry. Relat. Phenom. 4 (1974) 1. [4] R.F. Stebbings, Advan. At. Mol. Phys. 15 (1979) 77. [5] G.W. Foltz, C.J. Latimer, G.F. Hildebrandt, F.G. Kellert, K.A. Smith, W.P. West, F.B. Dunning and R.F. Stebbings, J. Chem. Phys. 67 (1977) 1352. [6] G.F. Hildebrandt, F.G. Kellert, F.B. Dunning, K.A. Smith and R.F. Stebbings, J. Chem. Phys. 68 (1978) 1349. [7] M. Matsuzawa, J. Phys. Sot. Jpn. 32 (1972) 1088. [8] H. Hotop and A. Niehaus, J. Chem. Phys. 47 (1967) 2506. [9] L.G. Christophorou, R.P. Blaunstein and D. Pittman, Chem. Phys. Letters 22 (1973) 41. [lo] H. Hiraishi and K. Kuchitsu, in: 10th International Conference on the Physics of Electronic and Atomic Collisions, Abstracts of papers, ed. G. Watel (NorthHolland, Amsterdam, 1978) p. 176. [ll] H. Hiraishi, T. Kondow, T. Fukuyama and K. Kuchitsu, J. Phys. Sot. Jpn. 46 (1979) 1628.
ionization
of highly
excited
neon atoms
387
1121 T. Shibata, T. Fukuyama and K. Kuchitsu, Bull. Chem. Sot. Jpn. 47 (1974) 2883. [13] D.S. Bailey, J.A. Hiskes and A.C. Riviere, Nuclear Fusion 5 (1965) 41. 1141 D.E. Stogryn and A.P. Stogryn, Mol. Phys. 11 (1966) 371. [15] G. Herzberg, Molecular spectra and molecular structure, Vol. 3, Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand, Princeton, 1967). [16] R.D. Nelson, D.R. Lide and A.A. Maryott, Selected values of electronic dipole moments for molecules in the gas phase, Standard Data Series (National Bureau of Standards, Washington, 1967). [17] K.P. Huber and G. Herzberg, Molecular spectra and molecular structure, Vol. 4, Constants of diatomic molecules (Van Nostrand, Princeton, 1979). [18] L. Sanche and G.J. Schulz, J. Chem. Phys. 58 (1973) 479. [19] S.Y. Chu, I. Ozkan and L. Goodman, J. Chem. Phys. 60 (1974) 1268. [20] K.D. Jordan and P.D. Burrow, Accounts Chem. Res. 11 (1978) 341. [21] L.G. Christophorou, G.S. Hurst and A. Hadjiantoniou, J. Chem. Phys. 44 (1966) 3506. [22] Y. Itikawa, J. Phys. Sot. Jpn. 30 (1971) 835. [23] Y. Itikawa, J. Phys. Sot. Jpn. 31 (1971) 1532.
COLLISIONAL BY NONPOLAR
IONIZATION
OF HIGHLY
Masashi UEMATSU, Hisato HIRAISHI, and Kozo KUCHITSU Deparfment of Chemistry, Faculty of Science, Hongo, Bunkyo-ku, Tokyo 113, Japan
Received
6 July
EXCITED
NEON
ATOMS
HYDROCARBONS
The
Tsutomu University
FUKUYAMA*,
Tamotsu
KONDOW
of Tokyo,
1981
When benzene, acetylene and ethylene were allowed to collide with Ne Rydberg atoms (17 d n s 40), significant Ne+ ions were observed. Their cross sections were estimated, with reference to the ionization by HzO, to be as large as 10~‘4-10~‘s cm’. However, no Nef signals were observed when ethane, methane and N2 were used as targets. Theoretical estimates of the cross sections for ionization by interaction of the quadrupole moment and polarizability of the first three molecules are several orders of magnitude smaller than the experimental cross sections given above. A by Christophorou et al., similarity of this phenomenon to the scattering of a thermal electron by benzene etc., observed is suggested.
1. Introduction
Recent experimental [l, 21 and theoretical [3] studies have shown that collisional ionization of a highly excited Rydberg atom by a polar molecule can be understood as scattering of the Rydberg electron in the highly excited atom by the target molecule [4]. The Rydberg electron causes rotational deexcitation of the molecule by interaction with the target dipole, and it is ejected from the atom by taking up the released rotational energy. Even if the target is nonpolar, a Rydberg atom can still be ionized if the target molecule has a positive electron affinity, because the Rydberg electron can be captured by the target [4-71. Sulfur hexafluoride is the best known example. To the contrary, there seems to have been no experimental evidence for the ionization of a Rydberg atom by a nonpolar target having a
negative electron affinityt. Hotop and Niehaus [8] attempted to detect the Ar+ ions resulting from the argon Rydberg atom colliding with a number of “nonpolar” molecules (Hz, NZ, O2 and CH& but they could not observe any noticeable ion signals. A theoretical explanation was given by Matsuzawa [3]. He calculated the cross section for ionization of a Rydberg atom by interaction with the quadrupole moment of a “nonpolar” molecule, such as Nz, and he estimated the cross section to be several orders of magnitude smaller than that caused by the dipole interaction of a polar target such as H20. In spite of these circumstances, it seemed questionable to conclude that all the “nonpolar” molecules were very poor ionizers of a Rydberg atom. In our attempt to find a suitable target, a guiding principle was provided by the similarity of a Rydberg electron to a free electron with thermal energy [3,4]. t A great
* Present Studies,
address: Ibaraki
National Institute 305, Japan.
0301-0104/82/0000-0000/$02.75
for
Environmental
study
@ 1982 North-Holland
majority
of the nonpolar
molecules,
negative electron affinities, are represented as “nonpolar”
molecules.
which have in the present
382
M.
Uematsu
et al. / Collisional
ionization
According to the electron swarm experiments of Christophorou et al. [9] for a number of gas molecules containing r-electrons, they conclude that the mobility of thermal electrons decreases, i.e. the scattering cross section of thermal electrons increases, as the number of the doublyoccupied r-orbitals in the target molecule increases. For instance, benzene was estimated to have a sizable scattering cross section of thermal electrons [9]. Therefore, benzene was first chosen as a test target in our study. A preliminary experiment indicated that benzene could indeed ionize a Rydberg atom [lo]. A subsequent refinement of the apparatus and experimental procedures has resulted in the present report. Collisional ionization of neon Rydberg atom, Ne**, with benzene and a few other target molecules, M,
(1)
Ne**+M+Ne’+M+e,
was observed. Neon was chosen because it was then possible to use its metastable (3P0,2) atoms, Ne”, for checking the effect of polar impurities by Penning ionization, Ne*+M+Ne+M++e.
of highly
excited
neon
atoms
chamber, which had an open bottom. This gauge was calibrated against an MKS pressure gauge (220B). The Ne+ signal intensity was found to be proportional to the target pressure. The relative cross sections were estimated from the slope of this linear relation. 2.2. Ionization
of Ne ** by nonpolar molecules
Significant Ne’ signals were observed when benzene, acetylene and ethylene were used as targets. The estimated upper and lower bounds of the cross sections for ionization, normalized to the ionization cross section for H20, are listed in table 1. On the other hand, when ethane, methane and nitrogen were used as targets, no Ne+ signal exceeding the noise level was observed. The main precursor of the observed Ne’ ions was proved to be Ne** by the following
Table 1 Observed
(2)
and calculated
cross
Observed d, relative d)
sections
a)
Calculated
‘)
2. Experimental
and results Hz0
2.1. Apparatus
C&s C2H2
The details of the apparatus and the experimental procedure have been described in our previous paper [ll]. Neon gas was introduced into the excitation region and was bombarded by an electron beam (50 eV). Charged particles were deflected from the neon beam by’an electrostatic field of about 200 V/cm, which corresponds to the ionization threshold for n -40; neutral species containing Rydberg atoms and metastable atoms, Ne*(3P2,0), were thereby allowed to enter the collision chamber. The pressure of the target gas in the collision region was typically 1 x 10e4 Torr. The ions formed in the collision were focused by ion lenses and mass-analyzed by a quadrupole mass filter. The target pressure was measured by an ionization gauge installed beneath the collision
C2H4 GH6 CH4
N2
1 2x10-z-4x10-3 1 x 1o-2 - 3 x 10m3 5x10-4-2x10-4 not observed e, not observed ” not observed ‘)
3.6 x 10-l’ 1.3xlo-‘s 3.6x lo-l6 5.4x10-17h) 4.1 x lo-l8 0
2.2 x lo-‘6
d)
relative
Cm2
f*g)
g.9
1
3.6 1.0 1.5 1.3 0 6.1
x x x x
lo-’ 1o-4 1o-5 1o-6
x lo-’
a) Cross sections for ionization of Ne** Rydberg atom. s) Estimated upper and lower bounds are listed. ‘) Ionization by quadrupole interaction estimated by use of the quadrupole moments [14] and the rotational constants [15]; see text. d, Normalized to the cross section for HzO. e, Smaller than 1 x 10m4. ” Ionization by dipole interaction, calculated by use of the dipole moment, 0.727 au [16]. ” These values are consistent with those estimated in ref. [3]; note, however, that no direct comparison can be made, because the n-distributions assumed in ref. [3] and in the present study are different. h, Crude estimate based on the symmetric-top approximation. i, Based on the rotational constant taken from ref. [17].
M. Uematsu
et al. / Collisional
ionization
measurements. Benzene was used as a target, since benzene gave the highest Ne+ intensity among the target molecules studied. (a) Excitation function: The Ne+ intensity was measured by varying the bombarding electron energy from 10 to 150 eV. The electron energy scale was calibrated with reference to the appearance potential of the Ne*(3P2,0) atoms, 16.6 eV, as measured by the &Hi ions formed by Penning ionization (2). The observed dependence of the Ne’ intensity on the electron impact energy is shown in fig. la. The onset of the Net signal is found to be 21.4kO.3 eV, which is nearly equal to the ionization potential of Ne, 21.56 eV. In addition, the curve shown in fig. la has a shape nearly identical with the excitation function of Ne** (fig. lb) determined from the collisional ionization (l), where M = HZO. (b) Field ionization: A field ionizer, composed of a pair of parallel-plate electrodes [ 111 with a spacing of 4 mm, was placed between the excitation region and the collision chamber. The Ne+ intensity was measured as a function of the electrostatic voltage applied to the electrodes. When the field strength was varied from 0 to 7.5 kV/cm, a uniform decrease in the Ne’ intensity was observed (see fig. 2). This dependence was ascribed to the loss of Ne** from the gas beam by field ionization. This measurement was used to make a crude estimate of the distribution of the principal
of highly
excited
neon
atoms
383
quantum number, n, of Ne** in the collision region [12]. On the basis of the calculation by Bailey et al. [13] on the critical field strength for ionization of H** atoms, the n distribution was estimated to range from about 17 to 40 with a peak at about 25. 2.3. Check of impurity
The benzene sample was of spectral grade (Nakarai Chemicals Ltd.) with quoted purity of 99.93%. Since a polar molecule ionizes Ne** much more efficiently than benzene, the presence of polar impurities was examined. The only impurity found in a gas chromatogram was toluene, the concentration being estimated to be 0.06%. The amount of thiophene was estimated to be 0.2 ppm by a calorimetric analysis. In order to remove water arising from ambient humidity, molecular sieve was placed in the sample holder, and the inlet copper tube which led the sample gas into the collision chamber was evacuated at about 150°C before the experiment. The concentrations of polar impurities were further examined by observation of impurity ions with the aid of a quadrupole mass spectrometer. When the benzene sample was introduced into the collision chamber, ions with masses 18 (H20’) and 92 (&H&H:) were observed in addition to Nef and several ions originating from C6H6, but no ions ascribable to
.
*. a**
a+
H20-N;+ .
(b)
.
.
.
Hz0 + c
.
0
0 .
l
,
,
1
,
50
Electron
,
,
,
1
,
100 Energy/eV
Fig. 1. The intensity of Ne+ produced in the collision function of the electron energy bombarding Ne.
.
.
.
.
I
1.
50
150
Electron of Ne**
with
(a) C6H6
#
.
*
I
.
100 Energy/eV
and (b) HZO,
plotted
.
.
.
1
150
in arbitrary
units
as
a
M.
384
2
Uematsu
et al. / Collisional
tc-+c&’ Ne’+ %“6+
=
ionization
of highly
excited
neon atoms
trap distillation. Water impurity was examined by the method described above. For acetylene (Matheson, purity 99.8%, the remaining substances being oxygen and nitrogen molecules) no impurity was observed in the mass spectrum.
3. Discussion
Fig. 2. The intensity of Net produced in the collision of Ne** with C6H6, plotted in arbitrary units as a function of the electrostatic field strength. The principal quantum numbers, n, which correspond to their critical field strengths, are estimated approximately [ 121 and indicated on the abscissa.
thiophene or any other impurity were observed. The H20f and C7Hi ions were produced by Penning ionization of the impurities [process (2), where M = HZ0 or ChHsCH3]. These impurities should also produce Ne’ ions by collisional ionization (l), which takes place simultaneously with the Penning process (2). The contribution of these polar impurities to the Ne+ signal were estimated in the following way. In a typical run, the total Net signal was 2000 cpm, while the HzOC signal was 30 cpm. When pure water was introduced into the collision chamber, the intensity ratio of the Ne’ and HzOC ions produced by processes (1) and (2), respectively, was 3.3. Therefore, the intensity of Ne+ due to the water impurity in the benzene sample was estimated to be about 100 cpm, namely 5% of the total Ne’ signal. Similarly, the contributions from toluene and thiophene were estimated to be 0.2% and less than 0.2%, respectively. Thus the polar impurities can account for only about 5% of the total Ne’ intensity observed; the remaining part of the Ne’ intensity is interpreted as originating from the collisional ionization by benzene. All the other gases used as targets, except for methane and nitrogen, were purified by trap to
The present study shows that three nonpolar molecules, benzene, acetylene and ethylene, have appreciable cross sections for ionization of neon Rydberg atoms. The cross sections for benzene and acetylene are estimated to be as large as 10-‘4-10-‘5 cm2 and are only two or three orders of magnitude smaller than that by a typical polar molecule, water. On the other hand, the cross sections for ionization by ethane, methane and nitrogen are all too small to be measurable, possibly less than 1 x lo-i6 cm*. The origin of the interaction causing ionization is yet uncertain; none of the following three mechanisms seems to contribute sufficiently to the cross sections. (1) Quadrupole interaction: According to Matsuzawa’s theory, the cross section for ionization by N2 due to quadrupole interaction is several orders of magnitude smaller than that by HZ0 [3]. In order to examine whether the same rule applies to the other targets, the cross sections for ionization by interaction of a quadrupole moment of the target were calculated. His formula for a linear molecule was used for NZ and C2H2. An analogous formula for a symmetric-top molecule was derived in the present study (see appendix) and applied to C6H6 and C2H6. The same equation was also applied to C2H4, which was approximated as a symmetrictop molecule with the rotational constants A and (B + C)/2; the quadrupole moment, Q,, was assumed to be the zt component of the quadrupole tensor. Because of these simplifications, the cross section for CZH4 is only an order-of-magnitude estimate. The molecular constants used in the present calculations were taken from refs. [14-171. The cross sections thus estimated were normalized to that by water calculated by the use of Matsuzawa’s
M.
Uematsu
et al. / Collisional
formula [3]. As shown in table 1, the calculated relative cross sections for ChH6, &Hz and C2H4 are several orders of magnitude smaller than the corresponding experimental estimates. Thus it is concluded that the quadrupole interaction is not a major mechanism of the collisional ionization by any of these molecules. (2) Polarization: Rydberg atoms can also be ionized by an induced dipole moment of the target. When this polarization effect is taken into account, the cross section for a symmetrictop molecule is given by the cu’-dependent terms in eq. (A.7). An order-of-magnitude estimation of these terms gives a cross section which is at most comparable with the contribution from the quadrupole interaction. A similar conclusion is reached for linear molecules. Hence, the effect of polarization of the target cannot remove the large discrepancy between the observed and calculated cross section by the three molecules. (3) Temporary negative-ion formation: If the Rydberg electron attaches to the target molecule forming a temporary negative ion and rapidly undergoes autodetachment, then the collisional ionization may have a large cross section. However, this possibility can be ruled out because the vertical electron affinities reported for benzene, acetylene and ethylene are -1.14 [18], -2.7 [19] or -2.6 [20], and -1.76 [18] eV, respectively, whereas the kinetic energy of the Rydberg electron is only about 10 meV, being insufficient for such an attachment process. 4. Concluding +electrons
remarks: Interaction
involving
ionization
of highly
excited
neon atoms
38.5
reported to be much larger than those by methane, nitrogen and ethylene [21]. Our present observation seems to support the prediction that “nonpolar” polyatomic molecules containing r-electrons should be good scatterers of a Rydberg electron as well as a thermal electron. Since nitrogen has a negligible cross section, either the molecular size or the vibrational degrees of freedom, or both, seem to contribute to the cross section. A further study is being undertaken for a more quantitative correlation of the cross sections for ionization with the mobilities of thermal electrons and for a better understanding of the mechanisms of these processes.
Acknowledgement
The authors are indebted to Drs. Y. Itikawa, M. Matsuzawa and I. Shimamura for their helpful suggestions, particularly in regard to the calculations given in the appendix. The present study has been supported by a grant from the Mitsubishi Foundation.
Appendix. Cross section for ionization of a Rydberg atom by the quadrupole moment of a symmetric-top molecule
The cross section for collisional ionization, A**(n)+B(JK)+A++B(J’K’)+e(&),
64.1)
has been given by Matsuzawa [3] as (T(n, JK + E, J’K’) k+k’
The observed cross sections for benzene and acetylene are two orders of magnitude larger than those for ethane and methane. This indicates that the ionization of a Ryberg atom takes place much more easily in the presence of r-electrons in unsaturated hydrocarbon targets. As mentioned in section 1, the similarity of the scattering of a Rydberg electron to that of a thermal electron is suggestive. The scattering cross section of a thermal electron by benzene is
2q.L2 =2 k
dFn(Q, E) I
de
Ik-k’l
x 1fe(JK -+J’K’))‘C? dQ, in terms of the following J, K: rotational quantum B, E: energy of the ejected CL: reduced mass of the
64.2)
symbols: numbers of molecule electron, colliding system,
M
386
Uematsu
et al. / Collisional
k and k’: wave numbers of the relative motion
before and after the collision, dF,(Q, E)/d& : form factor density per unit range of the ejected electron energy at momentum transfer Q; the binary-encounter theory gives eq. (14) of ref. [3],
of highly
excited
neon
atoms
described in ref. [3], it is shown that a(n ; JK + J’K’) 2K2(J2 -K2) -p, II2 15 0E [ J(J + l)(J - 1)(2J + 1)
=-4&r
(J2 -K2){(J - 1)’ -K2} +J(J-1)(2J+1)(2J-1)
dF,,(Q, E)/dE = (29/3&) x Q’/{(Q’-2~
ionization
- l/r~~)~+4Q~/n~}~,
fe(JK +J’K’): scattering amplitude tron scattering process,
(A.3)
for the elec-
1
~o)+&~Qp~'Lqh so)
x [Q;J,,(n,
41,
+&~2~‘2~4,h
(A.7)
where e +B(JK) + e’+ B(J’K’).
These formulas have been used for calculation of the ionization cross sections for a polar symmetric-top molecule and for a nonpolar linear molecule with a quadrupole moment (see eqs. (lob) and (12) of ref. [3]). In the present study, the above theory has been extended to the case of a nonpolar symmetric-top molecule with a quadrupole moment Q,. For this purpose, use is made of eq. (2.16) of ref. [22], which leads to lfel* = (klk’h,(JK 2J’+l
ZZ-
?r 00
J (K
+J’K’) J’ -K’
1 2 -m > 2
X
dr r2 utm(r)
II
idQr) ,
(A.3
0
where qf,,, is the differential cross section expressed in terms of the 3-j symbol, UL,,,is the asymptotic form of the potential, and il(Qr) is the spherical Bessel function. Becausesthe interaction is caused by a quadrupole moment, 2 is taken as 2. Then the selection rule for deexcitation is AJ = -1 and -2 [(2.7a) of ref. [22]]. The m = 0 case can be treated as follows. When the polarization interaction is taken into account, the potential is given, following eq. (3.1) of ref. [23], as zIzo(r) = -(47r/5)“2Qpr-3
cc
(A.4)
- (r/5)1’2a’r-4,
(A.6)
where (Y’ is the anisotropy of the polarizability of the molecule. Following the procedure
Ip,(n,4 = I [dFn(Q,41dElQp dQ,
64.8)
0
E is the energy of the relative motion, and e. is the difference between the transition energy and the binding energy. The first term in the second brackets represents the quadrupole interaction, while the second and third terms are due to the polarization interaction. The n-dependence of the cross section is obtained by taking the average over the thermal rotational distribution. The cross sections calculated in this way are further averaged over the experimental n-distribution function, estimated as described in section 2.2. The cross sections thus obtained are listed in table 1. Since benzene has a six-fold axis and ethane has a three-fold axis of symmetry, their quadrupole moments are given by independent scalars [14]. Therefore, the m=O case is sufficient for calculation of the cross sections. The above eq. (A.7) is also applicable to a nearly symmetric-top molecule such as ethylene. However, since ethylene has two independent components representing the quadrupole moment tensor [14], the case for m # 0 should also be considered. A numerical calculation for m # 0 is hampered by the lack of reliable experimental or theoretical values of these tensor components in addition to the complexity of the equation. Nevertheless, an estimate based on the m = 0 case, listed in table 1, seems to give a reasonable order of magnitude of the cross sections.
M.
Uematsu
et al. / Collisional
References [l]
M. Matsuzawa and W.A. Chupka, Chem. Phys. Letters 50 (1977) 373. [2] F.G. Kellert, K.A. Smith, R.D. Rundel, F.B. Dunning and R.F. Stebbings, J. Chem. Phys. 72 (1980) 3179. [3] M. Matsuzawa, J. Electron Spectry. Relat. Phenom. 4 (1974) 1. [4] R.F. Stebbings, Advan. At. Mol. Phys. 15 (1979) 77. [5] G.W. Foltz, C.J. Latimer, G.F. Hildebrandt, F.G. Kellert, K.A. Smith, W.P. West, F.B. Dunning and R.F. Stebbings, J. Chem. Phys. 67 (1977) 1352. [6] G.F. Hildebrandt, F.G. Kellert, F.B. Dunning, K.A. Smith and R.F. Stebbings, J. Chem. Phys. 68 (1978) 1349. [7] M. Matsuzawa, J. Phys. Sot. Jpn. 32 (1972) 1088. [8] H. Hotop and A. Niehaus, J. Chem. Phys. 47 (1967) 2506. [9] L.G. Christophorou, R.P. Blaunstein and D. Pittman, Chem. Phys. Letters 22 (1973) 41. [lo] H. Hiraishi and K. Kuchitsu, in: 10th International Conference on the Physics of Electronic and Atomic Collisions, Abstracts of papers, ed. G. Watel (NorthHolland, Amsterdam, 1978) p. 176. [ll] H. Hiraishi, T. Kondow, T. Fukuyama and K. Kuchitsu, J. Phys. Sot. Jpn. 46 (1979) 1628.
ionization
of highly
excited
neon atoms
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1121 T. Shibata, T. Fukuyama and K. Kuchitsu, Bull. Chem. Sot. Jpn. 47 (1974) 2883. [13] D.S. Bailey, J.A. Hiskes and A.C. Riviere, Nuclear Fusion 5 (1965) 41. 1141 D.E. Stogryn and A.P. Stogryn, Mol. Phys. 11 (1966) 371. [15] G. Herzberg, Molecular spectra and molecular structure, Vol. 3, Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand, Princeton, 1967). [16] R.D. Nelson, D.R. Lide and A.A. Maryott, Selected values of electronic dipole moments for molecules in the gas phase, Standard Data Series (National Bureau of Standards, Washington, 1967). [17] K.P. Huber and G. Herzberg, Molecular spectra and molecular structure, Vol. 4, Constants of diatomic molecules (Van Nostrand, Princeton, 1979). [18] L. Sanche and G.J. Schulz, J. Chem. Phys. 58 (1973) 479. [19] S.Y. Chu, I. Ozkan and L. Goodman, J. Chem. Phys. 60 (1974) 1268. [20] K.D. Jordan and P.D. Burrow, Accounts Chem. Res. 11 (1978) 341. [21] L.G. Christophorou, G.S. Hurst and A. Hadjiantoniou, J. Chem. Phys. 44 (1966) 3506. [22] Y. Itikawa, J. Phys. Sot. Jpn. 30 (1971) 835. [23] Y. Itikawa, J. Phys. Sot. Jpn. 31 (1971) 1532.