Electron-impact dissociation of N2: Angular distributions of highly excited Rydberg atoms

Chemical Physics 85 (1984) 40%412North-Holland, Amsterdam

:

_;

ELECTRbN-IMi?A& DIi3SOCIAiIilN OF N2: ANGULAR DISTRiBUTIONS OF HIGHLY EXClTED

__ -403

v RYDBERG Ai’OMS

Shigeru OHSHIMA, Tamotsu KONDOW, Tsutomu FUKUYAMA

’ and Kozo KUCHITSU

Department of Chemistv, Faculty of Science, The Uniwrsity of Tokyo, Hongo. Bunkyo- ky Tokyo 11.3, Japan and Institute for MolecuIar Science. Myodagi. Okaxki 444. Japan Received

28 October

1983

A nozzle beam of nitrogen molecules was excited by electrons (So-100 ev), and TOF spectra of high-Rydberg nitrogen at 60-120° with respect to the direction of the electron beam. The atoms, N**[lS 5 n s 45, n(peak) -191. were measured measured TOF and angular distributions were interpreted on the basis of Dunn’s selection rules and the core-ion model as from the molecular Rydberg states, Nt*, follows. At impact energy of = 60 eV or higher, N** atoms are mainly produced which converge to the ground and excited electronic states of Nf+, located at = 43-60 eV above the ground state of N, moIecufes in the Franck-Condon region. N** atoms with released kinetic energy, E; 5 7.5 eV, which show essentially angle-independent distributions. are mainly produced from the Rydberg states of Nt* converging to the X ‘2: state of Nz+. state as whereas N** atoms with EI, 2 7.5 eV. whose distributions have a distinct peak at 90’. are produced from the b’ff, well as the X *Za” state.

1. Introduction

Electron-impact processes of molecular nitrogen, Nz, have been studied extensively because of their importance in upper-atmosphere reactions [l] and gas discharges, where N2 is subjected to collisions with energetic electrons of more than 100 eV. At an excitation energy of above 40 eV, many excited states of the doubly-charged molecular ion, N:+, have been predicted by theoretical studies [2,3], and some of them have indeed been confirmed experimentally by Auger electron spectroscopy [4], double charge transfer spectroscopy [5], and photofragment spectroscopy [6]. However, none of the molecular Rydberg states converging to N;+ expected in this energy region have so far been identified. When a nitrogen molecule is excited to one of these Rydberg states, it is expected to dissociate rapidly and form a high-Rydberg nitrogen atom, N**, as suggested by Kupriyanov [7]. Such high-lying excited states cannot readily ’ Permanent address: tal Studies, yatabe,

The National Tsukuba-gun,

Institute for EnvironmenIbaraki 305, Japan.

be studied by conventional spectroscopy, but measurements of kinetic-energy and angular distributions of the fragment N** atoms provide useful information on the precursor Rydberg states; A careful measurement was made by Smyth et al. [S] on the kinetic energy distribution of N** atoms produced by electron impact on N, at a fixed detection angle of 90” with respect to the electron beam axis. Four dissociation limits were determined, and the highest one was suggested to be a high-Rydberg analog of the double ion process, N+( ‘P) + N”(3P)HR which correlates with molecular Rydberg states converging to Nz+, where HR denotes a high-Rydberg electron_ Angular distributions of the ionic fragments produced by dissociative ionization of simple molecules such as H,, 0,) and CO, have been measured, and the symmetries of their dissociative states have been analyzed on the. basis of Dunn’s selection rules [9]. A typical example is the dissoeil ation by electron ,impact on H, producing H+ [lo-151. This method should also be applicable to the estimation of the symmetry of molecular Rydberg states of N2 from a measurement of the

0301-0104/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

404

angular distribution of the neutral N** fragments produced by dissociative excitation. The present study was undertaken for this purpose. The kinetic-energy and angular distributions of the N** atoms formed by % :

[N,]**

+N**+N(orN++e)

(1)

were measured_ The experimental results were analyzed with the aid of Dunn’s selection rules and the core-ion model for two-electron excitation [7.8]. in which the excited electrons in high-Rydberg states are treated as spectators in the dissociation. This analysis takes advantage of the potential-energy curves of the Ni_+ states derived from an ab initio calculation by Thulstrup and Andersen [2].

skimmer into chamber @~~&hich: is’ &Gduated byanother oil diffusion putip (600 C/i);>.T$e beam is collimated and admitted to chambki III, where it is excited by electron-impact. A detector-of highRydberg species produced is placed in chamber IV. Chambers III and IV are connected by flexible bellows, by which chamber IV can be rotated about the collision center making an angle 8 = 60-120” with respect to the electron beam axis. Both chambers III and IV are evacuated by use of a turbomolecular pump (250 t/s)_ A sputtering ionization pump (50 8/s) is also used to help pumping chamber III. Typical pressures during the experiment are about 1 x 10e4, 2 x 10e5, 1 x 10e6, and 5 X lo-’ Torr for chambers I, II, III, and IV, respectively.

2. Esperimental

_.L_ 7 7 Beunl source

L I. Generul design

Nitrogen gas was introduced into a nozzle beam source directly from a gas cylinder. The stagnation pressure of the sample gas was optimized to be = 1000 Torr. A supersonic beam was formed

The apparatus consists of four differentially pumped sections used for nozzle-beam production (I). collimation (II), excitation (III). and detection (IV). as shown in fig. 1. Chamber I is evacuated by an oil diffusion pump (1000 t/s). A molecular

(I)

(11)

cm)

(N)

through a nozzle with an opening diameter of 50 pm. The beam was collimated by a skimmer of 0.64 mm diameter. The nozzle-skimmer distance was adjusted so as to minimize the background gas in chambers III and IV. The divergence angle of the beam was measured to be = 2S0 by rotating the detector around B = 90”. The beam intensity was = 3 X 10” s-’ sr-’ at a stagnation pressure of 1000 Torr. A typical beam intensity relative to the background was = 2 at a distance of 45 cm from the collision region. 2.3. Electron beam

Fig. 1. A schematic diagram of the apparatus. A molecular beam from a supersonic nozzle is excited by an electron beam in collision chamber 11. High-Rydberg nitrogen atoms produced are introduced into a pre-ionizer and a high-Rydberg detector. The pre-ionizer discriminates high-Rydbrrg fragments having different principal quantum numbers.

An electron gun placed in chamber III was used for excitation of the target molecule. A schematic diagram of the gun is shown in fig. 1. A tip of LaB, was welded on a hairpin filament and was heated resistively. A typical range of electron current was lo-200 PA corresponding to an impact energy of 5-200 eV. The energy spread was estimated to be = 0.5 eV from the line profile of the excitation function for the formation of metastable

S. &s.&~

et al. / EIec:ron -impact &.sociation of- N2

argon atoms, Ar( 3P0 2)_ The divergence of the’electron beam was rest&ted to within 3’ by using ti pair of-apertures. An actual diameter of the electron beam was - 3 mm. 2.4. High-Rydberg

sk-

---my

I

e

Detect&

I

7

, Nozzle ‘-7

( I I

-

detector

High-Rydberg fragment atoms of nitrogen were detected by means of electric field ionization. A plate and a grid, both made of stainless steel and 4 cm wide along the path of the fragment atoms, were placed in parallel at a spacing of 2.8 mm, and a channel-plate multiplier was mounted on the other side of the grid (fig. 1). An electric field of = 17 kV/cm was applied between the plate and the grid. High-Rydberg atoms (with principal quantum numbers, n, ranging from about 15 to 45, see section 3.2) which reached the detector unit were ionized within 2 mm from an entrance slit placed between the plate and the grid; thus all the high-Rydberg atoms were detected by the multiplier. A pre-ionizer consisting of two parallel electrodes separated by 2.7 mm was placed in front of the high-Rydberg detector, and an electrostatic voltage was applied across the two electrodes for discriminating the high-Rydberg fragments with different n. The rz-distribution of the fragments was determined by changing the voltage from 0.1 to 1.7 kV. This voltage range corresponds to n = 15-45 [16]. 2.5. Time-of-flight

-----_-----

l-------I I

40%

measuretnet~t

Fig. 2 shows a schematic diagram of the measurement of time-of-flight (TOF) spectra. The electron beam was turned on for l-5 ps by a trigger pulse, which concurrently started an ORTEC timeto-amplitude converter (TAC). High-Rydberg fragments produced by pulsed electrons were allowed to travel 23.8 cm to the high-Rydberg detector, and the resulting negative output pulse was amplified and shaped for stopping the TAC. The output from the TAC was converted to a pulseheight distribution by a multichannel analyzer (Canberra No. 30). T J cycle was repeated at a frequency of 2.5-10 k*lz until noise fluctuations were suppressed to less than 5% of the overall

&

Const.Frac. Oiscrimiiator MCA

stop

I

Fig. 2. A schemaric diagram of the electronic circuits used for measurements of TOF. MCP is a micro-channel plate.

signal at a TOF peak. The data were accumulated typically for 3 x lo4 s. The sequence of the measurements was controlled by a Hitachi level 3 microcomputer. The TOF spectra were measured at different scattering angles, and an angular distribution was obtained for high-Rydberg fragments with a specified kinetic energy. The angle subtended by the high-Rydberg detector was = 0.6”. The collision region (3 mm X 5 mm’) was fully viewed by the detector, so that no correction for the collision volume was necessary. Angular resolution, iimited mainly by the divergence of the molecular beam, was = 2.5”. The angular positions were calibrated with reference to the molecular beam axis, because the apparatus was so adjusted that the 0 = 90” position coincided with the molecular beam axis.

3. Results 3.1. Excitation

function

In order to test the performance of the entire system, excitation functions and n-distributions were measured for the high-Rydberg fragments and compared with thqse reported in previous works. In the measurements of the excitation functions, the electron gun was operated in a continuous mode and the electron energy, E, was varied

S. Ohshima et al. / Electron-impact

406

N2

e

N”

dissociarion of N?

6-116'

Nz-NY") E=80 eV

40

i20 160 60 ELECTRON ENERGY /eV

200

Fig. 3. Excitation functions for the production of high-Rydberg fmgments from N, measured at three detection angles. The excitation functions are normalized so BS to make the penk values at E = 120 eV all equal. Typical error bars are shown.

15

20

PRINCIPAL

quantum numbers of the high-Rydberg fragments were not resolved. At this stage the kinetic energies of the fragments were also unresolved (see section 3.3). Fig. 3 shows the excitation functions of the highRydberg fragments measured at three different angular positions. No measurable change in the excitation function, except in its intensity scale. was observed when the detection angle was varied. As shown in fig. 3, the intensity first rises slowly at = 24 eV, increases rapidly at = 50 eV, and tends to reach a broad maximum at = 120 eV. The present excitation function agrees with that reported by Kupriyanov [7] and is consistent with the TOF-resolved excitation function measured at 90” by Smyth et al. [S]. The rapid increase near 50 eV has been interpreted as an opening of a new dissociation channel involving a doubly charged core-ion [S] (see section 4). from

20 to 200 eV. The principal

Signals from high-Rydberg nitrogen atoms, S(Y). were measured as a function of the applied voltage. V, across the pre-ionizer: S(V) decreased with increase in the voltage. The minimum field required to ionize a high-Rydberg nitrogen atom with n is given by 6 X lO”/~t~ (V/cm) [16]. Hence. the n-distribution, P(rz), is given by = -2.4

x lo9 (dS/‘dV)/n’.

I

30

QLJANTWI

35

NUMBER,

-So

n

Fig. 4. Distributions of principal quantum numbers of highRydberg fragments measured at E = 80 eV and at three detection angles. The derivatives of the ion current transmitted through the electrostatic field with respect to n are plotted. The distributions are normalized so as to make the values of n = 19 all equal.

Fig. 4 shows the n-distribution of the high-Rydberg N** atoms produced by impact of 80 eV electrons on N,. The shape of this n-distribution is found to be independent, within experimental error. of the detection angle and the impact energy when it is higher than 50 eV. The distribution ranges from n = 15 to 45 and has a peak at n = 19. The overall shape of the distribution is influenced by the n-dependence of the cross section for excitation and that of the radiative decay probability

1161. 3.3. Kinetic-energy distributions

3.2. n-distribution

P(n)

I

25

(2)

The TOF spectra of the high-Rydberg fragments produced from Nz were measured in the impact-energy range of 50-100 eV and at 8 = 90” (fig. 5). The observed TOF spectra, F(t), were transformed to the corresponding kinetic-energy distributions by using the relations given by Smyth et al. [S], F’( Ek) = 2F(r)r3/nrL’

(3)

S. Ohshimq et al. / ElectroT -impact dissocia&

60 &O 20 TIME OF FLIGHT/ps

80 L

Fig. 5. TOF spe&a of high-RydbergN fragmentsfrom N, measured at four electron impact energies and at B = 90°.

and E, = nzL’/t’,

(4)

where F’( Ek) is the kinetic-energy distribution, E, is the released kinetic energy defined as a sum of the kinetic energies of the two atomic fragments *, nz is the mass of the fragment atom, L is the path length (23.8 cm), and t is the time of flight. The measured distributions of the kinetic energy of high-Rydberg nitrogen fragments at 90° are shown in fig. 6. The error bars shown in the figure represent random errors, which are estimated to be = 5% of the peak intensity. The overall uncertainty, including various systematic errors, is estimated from repeated measurements to be = 6%. The uncertainty in the released kinetic energy is mainly ascribed to the time resolution of the TOF measurement, which is limited by the width of the electron beam pulse (= 3 ps). Hence, the relative error in the measured time scale is = 10% for a typical flight time of 30 p.s. The uncertainty in the flight path arising from the detector geometry (see section 2.4) is estimated to be only = 1% of the

* Even

if a part of the high-Rydberg

4(u:

O/ Nz.

nitrogen

atoms

is pro-

duced by a process involving autoionization. N, 5 Nf* +

N** +N+ +e, the energy taken by the autoionizing

electron

6

8

10

12 IL Ek /eV

16

16

20

22

2&

Fig. 6. Distributions of the released kinetic energy. E,, of high-Rydberg fragments derived from the TOF spectra shown in fig. 5. They are plotted in arbitrary but consistent units. The error bars indicate statistical uncertainties.

total flight path. In summary, the overall uncertainty in the measurement of E, is = 10%. At 90” and at E = 100 eV, there is one intense peak at E, = 8.5 eV. In addition, there is a small hump peaked at = 13 eV and a shoulder at = 7.5 eV. The E,-distribution is essentially the same for E = 150 eV. On the other hand, the distribution

8=90’

, 40

60 ELECTRON

,

,

100

80 ENERGY/eV

Fig. 7. Excitation functions for the production of high-Rydberg fragments with *e-e E, values mcastired at 90° derived from the TOF spectra shown in fig. 5. The data are plotted in arbitrary

but

consistent

units.

The

excitation

is estimated to be so small that the relative kinetic energy is essentially shared by the kinetic energies of the two nitrogen

E, = 5.3 eV and 8.5 eV are extrapolated

fragments.

broken

and

47.3 eV respectively lines.

-reported

functions

to the thresholds, in ref.. [8], as shown

for 44.1 with

S. Ohshima et al. / Elecrron-impact dissociarion of IV_

408

IK&)

62.

75’

\?1-

T

90’

105’

118’

*-+.p-+A

24 eV

\ -?

z

BP”-%,.

0

3

f.

6

12

16

Ek

_

-0 - Ek=53eV

0=62’

20

2L

/ ev

Fig. 8. Distributions of the released kinetic energy. E,. of high-Rydberg fragments measured at E = 80 eV and at three detection angles. The data are plotted in arbitrary but consistent units. Fig. 9. Angular distributions of high-Rydberg fragments with specified E, produced at E = 80 eV. The data are plotted in arbitrary but consistent units.

measured at E = 60 eV is significantly weakened in the region of E, 1 12 eV, and at E = 50 eV the main peak is shifted to 6.5 eV. The main features of these E,-distributions agree semi-quantitatively with those reported by Smyth et al. (see figs. 4 and 5 of ref. [S], where distributions for lower E, are also shown). The excitation functions measured at 90” and at E, = 5.3,. 8.5 and 16.0 eV are displayed in fig. 7. They appear to be consistent with figs. 6-8 of ref. E=SO eV

SO”

90”

E=80

120°

60”

90°

[S]. but a detailed study of their threshold behavior is outside the scope of the present experiment. 3.4. Angtdar distributions The kinetic-energy distributions depend not only on the impact energy but also on the angle of E=lOO eV

eV

120~

60”

900

1200

Fig. 10. Angular distributions of high-Rydberg fragments with three Et values measured at three electron are plotted in arbitrary but consistent units. Estimated ranges of statistical uncertainties are indicated_

impact

energies.

The data

S. Okhiti

tit al. /

Electron-impact

det_=tion,.as shown.in fig. .8.-In spite of the l&ited range, 60 G 8 & 1200, th& rrieas,vred-angulardistributions of :the highiR$dberg fragments show a characteristic dependknce on ,E,. As summarized in fig. 9, the .d&.ributio& measured at E ~80 eV appears to be practically angle independent for E, 5 7.5 eV, while it has a clear maximum at 9W for E, 2 7.5 .eV, and this peak intensity decreases uniformly with E,. Typical examples of the dependence of the angular distribution on the impact energy are shown in fig. 10. In summary, a peak at 8 = 90” appears when E 150 eV and E, ~7.5 eV.

dissociation if N2

-.

?

Franck-Condon

4. Discussion 4. I. Energetics of molecular dissociative states

According to Smyth et al. [8], the asymptotic limits of the dissociation potential related to process (1) (see section 1) corresponds to a high-Rydberg manifold converging to the N+( 3P) + N “( ‘P) configuration when the impact energy is higher than 40 eV (see fig. 11). The separated-atom energy of this manifold ranges from 35.4 eV [N(~s~P)+N+(~P)] to 38.8 eV [N*(3P)+N’(3P)] [8]. Since the density of the Rydberg states converging to N’(3P) is proportional to e-‘.‘, where l is the energy difference from the N+(3P) level, nearly all the states in this manifold have energies ranging between 38.7 and 38-S eV. This effective width is less than the energy resolution of the present measurement of the fragment kinetic energy (= 0.6 ev), and hence, the energy of the asymptotic limits can be taken as 38.8 et-. This estimate can be used for estimation of the of the dissociative molecular states in energy, E,, the Franck-Condon region. By conservation of energy in the dissociation process, E, can be estimated from the released kinetic energy, E,, of the N** atom as E,,

= E, + 38.8 eV.

(5)

Since the measured E, ranges from 5 to 20 eV, to fall within 43-60 eV. The calculated’ potential-energy curves (fig. 11) [2,3) suggest that in this energy region the dissociative E FC is estimated

INTERNUCLEAR

DISTANCE

I ii

Fig. 11. Theoretical potential-energy curves for Ni+ reported in ref. [2]. with a shift of + 8.4 eV in the energy scale so as to fit the e:lergy of the ground state of Nz’ to the experimental adiabatic appearance potential, 43.7 k0.2 eV 161.A number of optically forbidden states existing in this energy region are left out, because they are regarded as unimportant for the present discussion on the production of high-Rydberg fragments. The dissociation limit, 2N+(-‘P). at 38.8 eV is shown.

molecular states formed by electron impact on N, are the neutral and/or singly-charged high-Rydberg states converging to the N,2+ core-ion, as pointed out by Kupriyanov [7]. The sudden enhancement of the excit&ion function observed at = 50 eV (see section 3.1 and fig. 3), which indicates the opening of a new-channel in this region, is consistent with this interpretation [8] (see section 4.5). 4.2. Possible dissociative states The following analysis of our experimental data is based on the core-ion model (7,8], Dunn’s selection rules [9], and the-potential curve for the core ion, Nz+ , calculated by Thulstrup and Andersen [2] (see section 4.5 and-fig. 11). In the range-of 43-60 eV,- the potential curves of the follotiing

S. Ohshima et al. / Electron -impact dissociation of IV,

410

states intersect X ‘El,

b’II,,

the Franck-Condon ‘Ag. ‘x;(2),

e’S,’

region: and d’Zz.

Several triplet states also exist in this region. but they are disregarded because the cross sections for excitation to these optically forbidden states by impact of electrons of 80 eV or higher are estimated to be one order of magnitude smaller than those for optically allowed excitations 1171. The main dissociative states among the candidates listed above are selected by an analysis of the measured angular distributions of high-Rydberg fragments. 4.3. Relation between the ~wnntetr3, of dissociative states and the angular distribution of fragments Under the present experimental conditions. where a nitrogen molecule is excited by electrons of more than 40 eV. the following processes of two-electron excitation are expected to take place efficiently [7.8.18]: (a) two electrons are excited to high-Rydberg states, and (b) one of the electrons is excited to a highRydberg state and the other to the continuum. The angular distributions of the high-Rydberg nitrogen atoms produced by either one of these processes can be interpreted in the same way. as shown by the following discussion. According to Dunn [9], the transition probability for excitation of a diatomic molecule by electron impact to a dissociative state, such as case (a) mentioned above. depends on (i) the symmetries of the initial and final molecular states, (ii) the relative orientation of the moleclzlar axis to the electron beam axis, and (iii) the angular momentum of the scattered electron. Dunn gave a selection rule for the excitation at threshold when molecular orientations are either parallel with or perpendicular to the electron beam axis. In these cases the wavefunction of the scattered electron is considered to be a spherically symmetric s wave [19] and, hence, can be disregarded in the consideration of symmetry. If the ground state has xg’ symmetry, as is the case of

N,, Dunn’s selection rules are such that the dissociative states that can be excited should belong The to “9’ and 2: for the parallel orientation. dependence on the initial orientation is transmitted to the angular distribution of the fragment atoms, since the dissociation in general occurs in a time shorter than the period of molecular rotation. A “parallel” transition gives an angular distribution with maxima at 8 = 0” and 180°, whereas a “perpendicular” transition gives one with maxima at 90” and 270°. For a transition such as x:g’ + xi, both parallel and perpendicular transitions can occur with equal probability. As a result, the angular distribution should be isotropic. Therefore. the angular distribution of the fragments near threshold and this selection rule provide information on the possible dissociative states of the precursor_ When the incident electron energy is appreciably higher than the threshold energy and the Born approximation is applicable, the above discussion can be modified as follows. In this case the transition matrix contains a scalar product of the incident and scattered waves. exp(iK*r), where K represents the momentum-transfer vector, i.e. the difference between the wave vectors of the incident and scattered electrons, ki and k,, respectively. Thus Dunn’s theory is applicable with the modification that K is taken as the reference axis instead of the electron beam axis, ki, in the threshold case. A crude estimation of the directional distribution of K relative to ki can be made as follows. Let the scattering angle (between ki and k,) be OSand the angle between K and ki be j?. For small S,, it follows that [9] sinP=8,/[62+(l+6)~s~]“‘,

(6)

where (7) The distribution of 0, was not estimated in the present study. but the reported differential cross sections for the electron-impact excitation of N, to optically allowed states [ZO] and to 2: states [21] suggest that the distribution falls off rapidly with 8, and the distribution beyond f?, of several degrees

S. Ohshima et aL / EIectron -impact dissociation of N2

is inconsequential at impact energies of more than 40 eV above threshold. When the impact energy is 80 eV and the threshold is 40 eV, which are the typical experimental conditions of the present study, 6 is - 0.4. Based on these estimates of 6 and B,, the most probable value of /3 is estimated to be only a few degrees. In other words, the vector K is essentially parallel with ki under the present experimental conditions. This discussion is essentially unchanged even when the precursor is ionic (case b), because the electron is ejected predominantly along the K axis by the requirement of momentum conservation [9-111 so that K remains to be an approximate reference axis. In summary, Dunn’s theory can be applied to the present analysis of fragment angular distributions, in which the reference axis K can be taken, in good approximation, as the electron beam axis. 4.4. Assignment

of the dissociative states

As shown in figs. 9 and 10, the observed distributions of the “slow” high-Rydberg fragments, 5 ,( E, s 7.5 eV, are essentially angle independent. On the other hand, those of the “fast” fragments, E, 2 7.5 eV, have a pronounced peak at t? = 90” on top of an angle-independent distribution. These characteristics of the angular distributions are independent of the impact energy ranging from 60 to 100 eV or higher. According to Dunn’s selection rules [9] (see section 4.3). the states which result in isotropic angular distributions are El, while those which result in angular distributions with a maximum at 0= 90° are III,. Though A, states also provide distributions of the latter category, A, can be disregarded in the present discussion because the A,+X+ transition is optically forbidden and should have a much smaller cross section than that for II,-Z,+. The energies of the dissociative states in the Franck-Condon region, E,, are estimated from eq. (5) to be 43.8 ,( E, ,( 46.3 eV and E, 2 46.3 eV for the slow and fast fragments, respectively. By inspection of the potential-energy curves of N$+ shown in fig. 11, the precursor states which meet the above conditions on the energetics and symmetry are X ‘2: (angle independent) and b’TII,

(peaked at 90”). -Thus Rydberg N atoms with duced, at least mainly, berg states converging with E, 2 7.5 eV from b’II U as well as XIX+ 8’

411

we believe that the. high5 5 E, 5 715 eV are profrom the molecular Rydto Ni+ X ‘2: and those the states converging to

4.5. Potential-energy

curves of the dissociative states. Comparison with previous results

A recent study of Cosby et al. [6] gives support to our above argument. They estimated by photofragment spectroscopy of Nz+ the E, value for X ‘Zp’ (v = 0) to be 4.8 + 0.2 eV; the E, value for b’lT, still remains somewhat uncertain, but it is = 7 eV if their observed vibrational levels are assigned to u = 1,2,3. The most probable E, values can also be estimated from the energies of the X ‘Ez and b*II, states obtained by double charge transfer spectroscopy [5]; the use of eq. (5) leads to E, = 4.3 f: 0.5 and 6.5 f 0.5 eV, respectively. They are comparable with the estimates of Cosby et al. [6] and ours, but they seem to be slightly too low. Recently, the N+ fragment ion produced from N;+ was measured by angle-resolved mass spectroscopy [18]; Edwards and Wood suggested that ‘II”, ‘Z:p’, and ‘Ag states were involved in the dissociation of Ni+ leading to 2N+(3P). Their suggestion is also consistent with the results of the present study. According to a study of N,Z+ by photofragment spectroscopy [6], the b’II, state is predissociative. In comparison with the theoretical estimates of the potential-energy curves, one should note the following non-trivial inconsistencies. The calculated potential-energy curve for X ‘xl, shown in fig. 11 [2], is not repulsive in the Franck-Condon region, whereas the present study indicates that a repulsive part of the X ‘2: potential intersects the Franck-Condon region. Likewise, the potential for b’II, appears to have a steeper repulsive wall in the Franck-Condon region than that shown in fig. 11 in order that the perpendicular transition, which leads to a fragment distribution peaked at 90°, makes a significant contribution in a wide range of E, (17.5 eV) (E, 146.3 ev). The potential-energy curves calculated by Cobb et al. [3] deviate still further from the experimental

412

S. Ohshimo er al. / Electron -impact dissociation of AI,

estimates. In particular, the potential curve for ‘II lies below that for ‘2+ in the Franck-Condon” region. A further theoietical calculation with higher credibility will resolve these discrepancies.

also due Nakamura

to Professors R.N. Zare for helpful discussion.

and

Hiroki

References 4.6. Comparison wit!z dissociative ionization

In the present study the core-ion model [7.8] is applied to two-electron excitation processes for interpretation of the experimental results. According to this model, dissociation of a molecular high-Rydberg state closely parallels the dissociative ionization of N,. The angular distributions of the fragment N+ ions produced in the dissociative ionization of Nz have been measured by Kieffer and van Brunt [22] and by Crowe and McConkey [23] for various ion energies. The observed distributions are reported to be ‘*isotropic” (at E > 100 ev) or “nearly isotropic” (at E < IO0 ev). Available for a quantitative comparison with our present observations is the angular distributions for N + with E, = 5.8 eV produced by 60 eV electrons, displayed in fig. 8 of ref. 1221 and fig. 1 of ref. [23]. At 60” < 0 < 120”, this distribution is practically angle independent and appears to be quite similar to that of our “slow” N** fragments (figs. 9 and 10). As for the “fast” ionic fragments, however. they did not report any significant enhancement around 90°, which has been observed in the present study in the distribution of the fragments with E, 2 7.5 eV.

[I] DC. Cartwright. J. Geophys. Res. 83 (1978) 517. [2] EW. Thulstrup and A. Andersen, J. Phys. B8 (1975) 965. [3] M. Cobb. T.F. Moran and R.F_ Burkman. J. Chem. Phys. 72 (1980) 4460. [4] W.E. Moddeman. J. Chem. Phys. 55 (1971) 2317. [5] J. Appell. J. Durup, EC. Fehsenfeld and P. Foumier, J. Phys. 86 (1973) 197. [6] P.C. Cosby. R. Mbller

The authors are grateful to Professor Eizi Hirota and the members of his laboratory for their hospitality and technical assistance_ Thanks are

H.

Helm.

Phys.

Rev.

A28

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