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Rotational distributions of CO2+ (X̃ 2Πg) produced by electron-impact ionization of supercooled CO2
Volume 15 1,number 6
CHEMICAL PHYSICS LETTERS
ROTATIONAL DISTRIBUTIONS OF CO; (3 ‘Q) PRODUCED BY ELECTRON-IMPACT IONIZATION Atsushi NAKAJIMA,
Takashi NAGATA,
Tamotsu
28 October 1988
OF SUPERCOOLED
KONDOW
CO2
and Kozo KUCHITSU
Department of Chemistry. Fuculfy of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Received 30 June 1988
Laser-induced fluorescence of CO: (2 ‘I’$) produced by electron impact on CO1 in a supersonic beam was measured. An analysis of the CO: (A-R) spectra has revealed that the rotational distribution of CO: is essentially Boltzmann and that the effective rotational temperature increases uniformly with decreasing impact energy, 26 + 1 K at 300 eV and 3 1f 1 K at 45 eV. This trend of rotational excitation, which is similar to that observed in the N,+e system, is interpreted in terms of the rotational transitions
in electron-impact
ionization.
1. Introduction Electron-impact ionization of simple molecules has been studied extensively. Absolute cross sections for ionization have been determined as a function of electron energy in a variety of molecular systems. Differential cross sections of secondary electrons have also been measured for several molecules. However, little is known about the details of this process, not even the vibrational and rotational state distributions of the nascent product ions. The only exception is the Nz+e system. Measurements of the Nl (B 2C: -X ‘Cz ) emission [ l-41 have shown that the product N2f (B ‘EC ) ion is rotationally more excited than the parent N, molecule and that the excitation is enhanced as the impact energy is lowered toward the ionization threshold. A similar trend of “rotational excitation” has also been observed in the N: (X ‘Cz ) state [ 5-9 1. In our previous study [ 91, the rotational excitation of N:(X) was measured quantitatively in the range of 25-300 eV by use of a supersonic molecular beam combined with laserinduced fluorescence. The observed trend of rotational excitation was interpreted qualitatively in terms of angular momentum transfer through multipole electron-molecule interactions as follows. The ionization process proceeds initially as an interaction between the incident electron and the target molecule, and then as an interaction between the 0 009-2614188 /$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
scattered and/or ejected electrons and the residual ion [ 5,6]. The electrons transfer a part of their angular momenta to molecular rotatioh through these interactions and, consequently, the rotational quantum number changes from N in the neutral state to N” in the ionic state. As the incident electron energy is lowered, the contribution of multipole interactions increases its importance and enhances the angular momenta transferred to the molecular rotation. Hence, a larger change in the rotational quantum number, 1mI - (N” -NI 2 2, occurs at a lower incident electron energy. This scheme seems to have general applicability so that one can expect to observe similar rotational excitation in other molecules. In order to test this expectation, the rotational distribution of CO: (x ‘&) produced in the reaction CO,(g
‘x,,‘)+e+CO:(jt211,)+2e
(I)
was examined in the present study by a measurement of iaser-induced fluorescence of CO: (A ‘l-I”% 211p). Since a supersonic nozzle was used to populate the target molecules in the lowest quantum levels, the product ions were also concentrated in a restricted number of rotational levels with small N” . The detection
sensitivity
was thus made sufficient
to
observe the LIF spectra under low-pressure conditions where the relaxation caused by molecular collisions could be disregarded. The rotational B.V.
511
Volume I5 1,number 6
28 October I988
CHEMICALPHYSICSLETTERS
distribution of the product ions was determined in the range of 45-300 eV. The observed trend of rotational excitation is discussed in comparison with the observations in the N2 + e system.
the LIF spectrum was free from collisional relaxation, which would occur before the ions were detected.
3. Results and discussion 2. Experimental The details of the experimental apparatus have been described previously [ 9 1. Pure CO1 gas stored at a pressure of 2 atm was expanded through a nozzle of 0.1 mm diameter in a pulsed mode. The duration of the pulsed beam was set to be z 100 ps fwhm at a frequency of ~4 Hz. The sample gas was condensed on a liquid nitrogen trap in order to keep the ambient pressure below 1.5~ 10m6 Torr. .The CO, beam was crossed by a pulsed electron beam ( % 1.5 ps duration and a current of % 15 pA) = 14 mm downstream from the nozzle, when the first 60 ps portion of the pulsed molecular beam came to the collision region. The electron energy was varied from 45 to 300 eV. The energy was calibrated against the appearance potential of the CO: (A-2) emission (17.6 eV [lo]), and the energy spread was estimated to be x 0.5 eV. The output pulse of a dye laser pumped by a XeCl excimer laser (Lambda Physik FL20WEMG102) was introduced into the collision region with a delay time of typically 0.5 ps after the electron pulse was turned off. The spontaneous emission from CO: (A) directly excited through the electron-impact ionization faded out almost completely during the delay time. The probe laser was tuned to excite the CO: (A *lJ,(OOO)tji 21&(OOO)) transition which corresponds to wavelengths around 350 nm. The laser-induced fluorescence was collected through a series of lenses and detected by a photomultiplier (Hamamatsu R1398). A band pass filter (1,,,1: 365 nm, fwhm = 8 nm) was used to isolate the CO: (A211,(OOO)+% *I&( 100)) emission from the stray light of the probe laser. The signal from the photomultiplier was amplified and digitally recorded by a transient digitizer (Iwatsu DM-2350, 50 MHz sampling rate) controlled by a microcomputer (NEC PC-9801 ). The LIF spectrum was measured by varying the delay time between the electron and the laser pulse in the range of 0.1-1.0 ps. The spectral features did not change with the delay time. It was evident that 512
A typical LIF spectrum of CO: produced at an impact energy of 200 eV is shown in fig. la. Each spectral line can be assigned to a rotational transition of the A *I& (000)-~2113irp(OOO) band. The R branch forms a band head around N” = 10. Rotational lines up to N” x22 were observed in the P branch, though not shown in the figure. The fluorescence assignable to the A ‘II, i2U(OOO)ii ‘l-I, ,2g(OOO) transition around 35 1 nm was also observed with an intensity comparable with that of the A 2113,2U-g *IIJjZg transition. The spectral features of the a= l/2 manifold were almost identical with those of the Q=3/2 manifold. The features of were found to depend on the the x 2IIZ,2U- x ‘l& electron impact energy, as shown in fig. 2. The relative intensity of the R head increased as the electron energy was lowered from 200 to 45 eV; namely,
the lower the electron impact energy the more CO: ions were populated in the rotational levels with higher N”. This trend of rotational excitation of the product ions with decreasing electron energy is simI 234
I
I
I
5
6
7
Jn-l/2
co: (2%3,,)
n
(a)
I
350.45
I
350.50
I
350.55
wavelength/nm
Fig. I. (a) Typical LIF spectrum of CO: (R 2~~~~,(000)) observed at 200 eV. (b) Calculated spectrum in the band-envelope simulation procedure (I;= 26 K).
Volume
15I, number 6
CHEMICAL
PHYSICS LETTERS
28 October
1988
tion characterized by an effective rotational temperature, TR, was assumed in the simulation. The band envelope was calculated by use of the bandwidth of the probe laser (~0.3 cm-‘), the rotational constants in the relevant states [ 111, and the rotational line strengths for saturated electronic transition [ 12 1. The best-fit band envelope thus calculated is shown in fig. lb for the impact energy of 200 eV. This simulated spectrum agrees well with the observed spectrum, but the line intensities of higher N” levels arc more enhanced than those expected from the assumed Boltzmann distribution. Nevertheless, the extent of such overpopulation does not exceed 2-3% of the total populations and, hence, this discrepancy results in only slight ambiguity in the estimation of T,.
I
I 350.5
350.4 wavelength/m
Fig. 2. Dependence of the spectral features on electron-impact energy: The spectral intensities are normalized at the maximum of the Q head in order to show the increase in the relative intensity of the R head with decreasing electron-impact energy.
ilar to that observed in the electron-impact ionization of Nz [9]. The rotational-state distributions of the CO: ions were determined from the LIF spectra by a band-envelope simulation in order to obtain a quantitative measure of the observed rotational excitation as a function of electron energy. A Boltzmann distribu-
The rotational temperatures determined at various impact energies are listed in table 1. The temperature varies from 26 + 1 to 3 1 f 1 K as the electron energy decreases from 300 to 45 eV, and the increase in TR is more significant as the electron energy is lowered below 100 eV. In the range of 45-300 eV, the CO: ions are formed in the A ‘II,, fi ‘E:,’ and c ‘.Zl states as well as in the x *I& state. Most of the ions prepared in these electronically excited states, with the exception of the predissociative c state [ 131, readily cascade radiatively to the ground state within the time period of the production and detection ( z 2 us) of the CO: ions. The radiative lifetimes are reported to be 124nsinA’TI,and 140nsinfi’C: [14].Consequently, the CO: (ii) ions detected under the pres-
Table 1 Degrees of rotational J% (eV)
excttation
in CO: (2 ‘II&
and NT (X ‘E:
) produced m electron-impact
CQZ
ionization
N2
Fa (R) 8)
Ea (meV) b,
45 60
31(l) 30(f)
100
28(l)
200 300
26(l) 26(l)
2.7(l) 2.6( 1) 2.4(l) 2.2(l) 2.2(l)
&i” 6.4(l) 6.3(l) 6.0( 1) 5.8(l) 5.8(l)
& (meV) c,
J$I?d)
3.6(I) 3.5(l) 2.6( 1) 2.4(2) 2.3(2)
3.4( I ) 3.3( 1 ) 2.8( 1) 2.7(2) 2.7(2)
” Effective rotational temperature. Digits in parentheses are absolute errors estimated from a band-envelope simulation procedure. b, The effective rotational temperature is converted to the average rotational energy, &, for comparison with the NT (X ‘2: ) case. ” From ref. [9]. Note that the rotational distributions ofNC cannot be represented by a single Boltzmann distribution; hence only the average rotational energy, &, is given. d, Average rotational quantum numbers calculated from P(W ) obtained in ref. [9].
513
Volume 15I. number 6
CHEMICAL PHYSICS LETTERS
ent experimental conditions include the ions cascading from the A ‘lIU and B ‘CL states in addition to those formed directly in the % 21’Igstate by electron impact ionization. The contributions of these cascading processes were roughly estimated from the reported total cross section for ionization of CO2 [ 151 and the cross sections for emission from CO: (A) and CO: (B) [16]; for example, at the impact energy of 100 eV at most 15% of the detected CO,’ (R) ions originates from the A ‘IIU state and N 35% from the B 2E;: state. These fractions are almost independent of the impact energy in the range of 45-150 eV. The finding that the observed rotational distribution is essentially Boltzmann can be interpreted as follows. The rotational distribution of CO: (%) ions cascading from the excited states, mainly from B ‘&+, is not too different from that of the CO: (%) ions primarily produced in the L%‘I$ state and, hence, the radiative cascading does not seriously alter the nascent rotational distribution in the z ‘II, state. In addition, a similar trend of rotational excitation is expected in both the g ‘C: and B 2Ez states when the electron energy is decreased. Otherwise, a bimodal rotational distribution, such as two Boltzmann distributions superimposed on each other, should have been observed in the present case. This interpretation is also supported by the experimental results obtained in the electron-impact ionization of NZ. The N: ions formed in X *C,+ show a rotational distribution that is almost identical with that in B ‘Z,’ [ 71, and nearly the same trend of rotational excitation was observed in both states [ $1, Although similar trends of rotational excitation were observed in the electron-impact ionization of N, and CO1, the degrees of enhancement in the rotational energy are quite different. In the case of CO:, the net increase in the rotational energy from 300 to 45 eV is x0.4 meV (5 K) while the corresponding value is 1.3 meV in the case of N: [ 91. The rotational excitation of the product ions is determined by the selection rules for rotational transitions. For comparison of the selection rules operating in these ionization processes, it seems suitable to use the average quantum number, instead of the rotational energy, as a measure of the rotational excitation. The average rotational quantum number of the product ions is defined as 514
28 October 1988
(2) where P(N” ) represents the relative rotational population at the N” rotational level normalized as &P(N” ) = 1. Here P(N” ) is assumed to be a Eoltzmann distribution with an effective rotational temperature of TR. The N” values were calculated to be 5.8 at 300 eV and 6.4 at 45 eV. Thus the CO: ions produced at 45 eV gain an angular momentum of ;~0.6fi on the average in excess of that of the ions produced at 300 eV. On the other hand, in the case of N: the P value varies from 2.7 to 3.4 when the impact energy is lowered from 300 to 45 eV (see table I ). Thus the net increase in the angular momentum is nearly equal to that for CO:. As discussed previously, transitions with an increase in the rotational quantum number from a neutral level N to an ionic level N” (IV” -N>O) are favored when N is small, since the ratio of the probability for N+N”=N+AN,AN>OtothatforN-+N”=N-m is expected to decrease with N, as is the case in molecular rotational transitions [ 9 1. Ions with low N” are produced from neutral molecules with low N. Therefore, when the electron energy is lowered the increment in Ntf becomes more prominent for product ions populated in rotational levels with lower N”. Because the P value of the CO: ions is larger than that of N: and N” increases nearly equally with decreasing impact energy in both systems, multipole interactions, which result in IANI 2 2, are likely to contribute more efficiently in CO: even at larger N” levels. The present study thus provides a semiquantitative support for the expectation that rotational excitation is a general trend in electron-impact ionization processes. A more quantitative discussion awaits a theoretical investigation on the cross section for ionization with a change in the molecular angular momentum.
References [ I] G. Gulp and A.T. Stair Jr., J. Chim. Phys. I (1967) 57. 121D. Coe, F. Robben, L. Talbot and R. Cattolica, Phys. Fluids 23 ( 1980) 706. 13I B.M. Dekoven, D.H. Levy, H.H. Harris, B.R. Zegarski and T.A. Miller, J. Chem. Phys. 74 (1981) 5659.
Volume
151, number 6
CHEMICAL
[4] S.P. Hemander, P.J. Dagdigian and J.P. Doering, Chem. Phys. Letters 91 ( 1982) 409; J. Chem. Phys. 77 (1982) 6021. [5] .I. Allison, T. Kondow and R.N. Zare, Chem. Phys. Letters 64 (1979) 202. [6] M.I. Lester, B.R. Zegarski and T.A. Miller, J. Phys. Chem. 87 (1983) 5228. [7] H. Helvajian, B.M. Dekoven and A.P. Baronavski, Chem. Phys. 90 ( 1984) 175. [ 81 H. Kawazumi, T. Uchida and T. Ogawa, Bull. Chem. Sot. Japan 59 (1986) 937. [9] T. Nagata, A. Nakajima, T. Kondow and K. Kuchitsu, J. Chem. Phys. 87 (1987) 6507.
PHYSICS LETTERS
28 October
1988
[lo] J.H.D. Eland and C.J. Danby, Intern. J. Mass Spectrom. Ion Phys. 1 (1968) 1 Il. [ 111 S. Morozowski, Phys. Rev. 60 (1941) 730. [ 121 W.H. Fisher, T. Canington, S.V. Filseth, C.M. Soclowski and C.H. Dugan, Chem. Phys. 82 (1983) 443. [ 131 J.H.D. Eland and J. Berkowitz, J. Chem. Phys. 67 (1977) 2782. [ 141 J.P. Maierand F. Tbommen, Chem. Pbys. 51 (1980) 319. [ IS] A. Crowe and J.W. McConkey, J. Phys. B 7 (1974) 349. [ 161 J.M.Ajello,
J. Chem. Phys. 55 (1971) 3169.
515
CHEMICAL PHYSICS LETTERS
ROTATIONAL DISTRIBUTIONS OF CO; (3 ‘Q) PRODUCED BY ELECTRON-IMPACT IONIZATION Atsushi NAKAJIMA,
Takashi NAGATA,
Tamotsu
28 October 1988
OF SUPERCOOLED
KONDOW
CO2
and Kozo KUCHITSU
Department of Chemistry. Fuculfy of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Received 30 June 1988
Laser-induced fluorescence of CO: (2 ‘I’$) produced by electron impact on CO1 in a supersonic beam was measured. An analysis of the CO: (A-R) spectra has revealed that the rotational distribution of CO: is essentially Boltzmann and that the effective rotational temperature increases uniformly with decreasing impact energy, 26 + 1 K at 300 eV and 3 1f 1 K at 45 eV. This trend of rotational excitation, which is similar to that observed in the N,+e system, is interpreted in terms of the rotational transitions
in electron-impact
ionization.
1. Introduction Electron-impact ionization of simple molecules has been studied extensively. Absolute cross sections for ionization have been determined as a function of electron energy in a variety of molecular systems. Differential cross sections of secondary electrons have also been measured for several molecules. However, little is known about the details of this process, not even the vibrational and rotational state distributions of the nascent product ions. The only exception is the Nz+e system. Measurements of the Nl (B 2C: -X ‘Cz ) emission [ l-41 have shown that the product N2f (B ‘EC ) ion is rotationally more excited than the parent N, molecule and that the excitation is enhanced as the impact energy is lowered toward the ionization threshold. A similar trend of “rotational excitation” has also been observed in the N: (X ‘Cz ) state [ 5-9 1. In our previous study [ 91, the rotational excitation of N:(X) was measured quantitatively in the range of 25-300 eV by use of a supersonic molecular beam combined with laserinduced fluorescence. The observed trend of rotational excitation was interpreted qualitatively in terms of angular momentum transfer through multipole electron-molecule interactions as follows. The ionization process proceeds initially as an interaction between the incident electron and the target molecule, and then as an interaction between the 0 009-2614188 /$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
scattered and/or ejected electrons and the residual ion [ 5,6]. The electrons transfer a part of their angular momenta to molecular rotatioh through these interactions and, consequently, the rotational quantum number changes from N in the neutral state to N” in the ionic state. As the incident electron energy is lowered, the contribution of multipole interactions increases its importance and enhances the angular momenta transferred to the molecular rotation. Hence, a larger change in the rotational quantum number, 1mI - (N” -NI 2 2, occurs at a lower incident electron energy. This scheme seems to have general applicability so that one can expect to observe similar rotational excitation in other molecules. In order to test this expectation, the rotational distribution of CO: (x ‘&) produced in the reaction CO,(g
‘x,,‘)+e+CO:(jt211,)+2e
(I)
was examined in the present study by a measurement of iaser-induced fluorescence of CO: (A ‘l-I”% 211p). Since a supersonic nozzle was used to populate the target molecules in the lowest quantum levels, the product ions were also concentrated in a restricted number of rotational levels with small N” . The detection
sensitivity
was thus made sufficient
to
observe the LIF spectra under low-pressure conditions where the relaxation caused by molecular collisions could be disregarded. The rotational B.V.
511
Volume I5 1,number 6
28 October I988
CHEMICALPHYSICSLETTERS
distribution of the product ions was determined in the range of 45-300 eV. The observed trend of rotational excitation is discussed in comparison with the observations in the N2 + e system.
the LIF spectrum was free from collisional relaxation, which would occur before the ions were detected.
3. Results and discussion 2. Experimental The details of the experimental apparatus have been described previously [ 9 1. Pure CO1 gas stored at a pressure of 2 atm was expanded through a nozzle of 0.1 mm diameter in a pulsed mode. The duration of the pulsed beam was set to be z 100 ps fwhm at a frequency of ~4 Hz. The sample gas was condensed on a liquid nitrogen trap in order to keep the ambient pressure below 1.5~ 10m6 Torr. .The CO, beam was crossed by a pulsed electron beam ( % 1.5 ps duration and a current of % 15 pA) = 14 mm downstream from the nozzle, when the first 60 ps portion of the pulsed molecular beam came to the collision region. The electron energy was varied from 45 to 300 eV. The energy was calibrated against the appearance potential of the CO: (A-2) emission (17.6 eV [lo]), and the energy spread was estimated to be x 0.5 eV. The output pulse of a dye laser pumped by a XeCl excimer laser (Lambda Physik FL20WEMG102) was introduced into the collision region with a delay time of typically 0.5 ps after the electron pulse was turned off. The spontaneous emission from CO: (A) directly excited through the electron-impact ionization faded out almost completely during the delay time. The probe laser was tuned to excite the CO: (A *lJ,(OOO)tji 21&(OOO)) transition which corresponds to wavelengths around 350 nm. The laser-induced fluorescence was collected through a series of lenses and detected by a photomultiplier (Hamamatsu R1398). A band pass filter (1,,,1: 365 nm, fwhm = 8 nm) was used to isolate the CO: (A211,(OOO)+% *I&( 100)) emission from the stray light of the probe laser. The signal from the photomultiplier was amplified and digitally recorded by a transient digitizer (Iwatsu DM-2350, 50 MHz sampling rate) controlled by a microcomputer (NEC PC-9801 ). The LIF spectrum was measured by varying the delay time between the electron and the laser pulse in the range of 0.1-1.0 ps. The spectral features did not change with the delay time. It was evident that 512
A typical LIF spectrum of CO: produced at an impact energy of 200 eV is shown in fig. la. Each spectral line can be assigned to a rotational transition of the A *I& (000)-~2113irp(OOO) band. The R branch forms a band head around N” = 10. Rotational lines up to N” x22 were observed in the P branch, though not shown in the figure. The fluorescence assignable to the A ‘II, i2U(OOO)ii ‘l-I, ,2g(OOO) transition around 35 1 nm was also observed with an intensity comparable with that of the A 2113,2U-g *IIJjZg transition. The spectral features of the a= l/2 manifold were almost identical with those of the Q=3/2 manifold. The features of were found to depend on the the x 2IIZ,2U- x ‘l& electron impact energy, as shown in fig. 2. The relative intensity of the R head increased as the electron energy was lowered from 200 to 45 eV; namely,
the lower the electron impact energy the more CO: ions were populated in the rotational levels with higher N”. This trend of rotational excitation of the product ions with decreasing electron energy is simI 234
I
I
I
5
6
7
Jn-l/2
co: (2%3,,)
n
(a)
I
350.45
I
350.50
I
350.55
wavelength/nm
Fig. I. (a) Typical LIF spectrum of CO: (R 2~~~~,(000)) observed at 200 eV. (b) Calculated spectrum in the band-envelope simulation procedure (I;= 26 K).
Volume
15I, number 6
CHEMICAL
PHYSICS LETTERS
28 October
1988
tion characterized by an effective rotational temperature, TR, was assumed in the simulation. The band envelope was calculated by use of the bandwidth of the probe laser (~0.3 cm-‘), the rotational constants in the relevant states [ 111, and the rotational line strengths for saturated electronic transition [ 12 1. The best-fit band envelope thus calculated is shown in fig. lb for the impact energy of 200 eV. This simulated spectrum agrees well with the observed spectrum, but the line intensities of higher N” levels arc more enhanced than those expected from the assumed Boltzmann distribution. Nevertheless, the extent of such overpopulation does not exceed 2-3% of the total populations and, hence, this discrepancy results in only slight ambiguity in the estimation of T,.
I
I 350.5
350.4 wavelength/m
Fig. 2. Dependence of the spectral features on electron-impact energy: The spectral intensities are normalized at the maximum of the Q head in order to show the increase in the relative intensity of the R head with decreasing electron-impact energy.
ilar to that observed in the electron-impact ionization of Nz [9]. The rotational-state distributions of the CO: ions were determined from the LIF spectra by a band-envelope simulation in order to obtain a quantitative measure of the observed rotational excitation as a function of electron energy. A Boltzmann distribu-
The rotational temperatures determined at various impact energies are listed in table 1. The temperature varies from 26 + 1 to 3 1 f 1 K as the electron energy decreases from 300 to 45 eV, and the increase in TR is more significant as the electron energy is lowered below 100 eV. In the range of 45-300 eV, the CO: ions are formed in the A ‘II,, fi ‘E:,’ and c ‘.Zl states as well as in the x *I& state. Most of the ions prepared in these electronically excited states, with the exception of the predissociative c state [ 131, readily cascade radiatively to the ground state within the time period of the production and detection ( z 2 us) of the CO: ions. The radiative lifetimes are reported to be 124nsinA’TI,and 140nsinfi’C: [14].Consequently, the CO: (ii) ions detected under the pres-
Table 1 Degrees of rotational J% (eV)
excttation
in CO: (2 ‘II&
and NT (X ‘E:
) produced m electron-impact
CQZ
ionization
N2
Fa (R) 8)
Ea (meV) b,
45 60
31(l) 30(f)
100
28(l)
200 300
26(l) 26(l)
2.7(l) 2.6( 1) 2.4(l) 2.2(l) 2.2(l)
&i” 6.4(l) 6.3(l) 6.0( 1) 5.8(l) 5.8(l)
& (meV) c,
J$I?d)
3.6(I) 3.5(l) 2.6( 1) 2.4(2) 2.3(2)
3.4( I ) 3.3( 1 ) 2.8( 1) 2.7(2) 2.7(2)
” Effective rotational temperature. Digits in parentheses are absolute errors estimated from a band-envelope simulation procedure. b, The effective rotational temperature is converted to the average rotational energy, &, for comparison with the NT (X ‘2: ) case. ” From ref. [9]. Note that the rotational distributions ofNC cannot be represented by a single Boltzmann distribution; hence only the average rotational energy, &, is given. d, Average rotational quantum numbers calculated from P(W ) obtained in ref. [9].
513
Volume 15I. number 6
CHEMICAL PHYSICS LETTERS
ent experimental conditions include the ions cascading from the A ‘lIU and B ‘CL states in addition to those formed directly in the % 21’Igstate by electron impact ionization. The contributions of these cascading processes were roughly estimated from the reported total cross section for ionization of CO2 [ 151 and the cross sections for emission from CO: (A) and CO: (B) [16]; for example, at the impact energy of 100 eV at most 15% of the detected CO,’ (R) ions originates from the A ‘IIU state and N 35% from the B 2E;: state. These fractions are almost independent of the impact energy in the range of 45-150 eV. The finding that the observed rotational distribution is essentially Boltzmann can be interpreted as follows. The rotational distribution of CO: (%) ions cascading from the excited states, mainly from B ‘&+, is not too different from that of the CO: (%) ions primarily produced in the L%‘I$ state and, hence, the radiative cascading does not seriously alter the nascent rotational distribution in the z ‘II, state. In addition, a similar trend of rotational excitation is expected in both the g ‘C: and B 2Ez states when the electron energy is decreased. Otherwise, a bimodal rotational distribution, such as two Boltzmann distributions superimposed on each other, should have been observed in the present case. This interpretation is also supported by the experimental results obtained in the electron-impact ionization of NZ. The N: ions formed in X *C,+ show a rotational distribution that is almost identical with that in B ‘Z,’ [ 71, and nearly the same trend of rotational excitation was observed in both states [ $1, Although similar trends of rotational excitation were observed in the electron-impact ionization of N, and CO1, the degrees of enhancement in the rotational energy are quite different. In the case of CO:, the net increase in the rotational energy from 300 to 45 eV is x0.4 meV (5 K) while the corresponding value is 1.3 meV in the case of N: [ 91. The rotational excitation of the product ions is determined by the selection rules for rotational transitions. For comparison of the selection rules operating in these ionization processes, it seems suitable to use the average quantum number, instead of the rotational energy, as a measure of the rotational excitation. The average rotational quantum number of the product ions is defined as 514
28 October 1988
(2) where P(N” ) represents the relative rotational population at the N” rotational level normalized as &P(N” ) = 1. Here P(N” ) is assumed to be a Eoltzmann distribution with an effective rotational temperature of TR. The N” values were calculated to be 5.8 at 300 eV and 6.4 at 45 eV. Thus the CO: ions produced at 45 eV gain an angular momentum of ;~0.6fi on the average in excess of that of the ions produced at 300 eV. On the other hand, in the case of N: the P value varies from 2.7 to 3.4 when the impact energy is lowered from 300 to 45 eV (see table I ). Thus the net increase in the angular momentum is nearly equal to that for CO:. As discussed previously, transitions with an increase in the rotational quantum number from a neutral level N to an ionic level N” (IV” -N>O) are favored when N is small, since the ratio of the probability for N+N”=N+AN,AN>OtothatforN-+N”=N-m is expected to decrease with N, as is the case in molecular rotational transitions [ 9 1. Ions with low N” are produced from neutral molecules with low N. Therefore, when the electron energy is lowered the increment in Ntf becomes more prominent for product ions populated in rotational levels with lower N”. Because the P value of the CO: ions is larger than that of N: and N” increases nearly equally with decreasing impact energy in both systems, multipole interactions, which result in IANI 2 2, are likely to contribute more efficiently in CO: even at larger N” levels. The present study thus provides a semiquantitative support for the expectation that rotational excitation is a general trend in electron-impact ionization processes. A more quantitative discussion awaits a theoretical investigation on the cross section for ionization with a change in the molecular angular momentum.
References [ I] G. Gulp and A.T. Stair Jr., J. Chim. Phys. I (1967) 57. 121D. Coe, F. Robben, L. Talbot and R. Cattolica, Phys. Fluids 23 ( 1980) 706. 13I B.M. Dekoven, D.H. Levy, H.H. Harris, B.R. Zegarski and T.A. Miller, J. Chem. Phys. 74 (1981) 5659.
Volume
151, number 6
CHEMICAL
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PHYSICS LETTERS
28 October
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