Autoionizing Rydberg states in Na2S. Leutwyler, T. Heinis, M. Jungen, H.P. Härri, and E. Schumacher Citation: The Journal of Chemical Physics 76, 4290 (1982); doi: 10.1063/1.443469 View online: http://dx.doi.org/10.1063/1.443469 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/76/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Predissociation and autoionization of triplet Rydberg states in molecular hydrogen J. Chem. Phys. 121, 3058 (2004); 10.1063/1.1773157 Autoionizing Rydberg states of NO in strong electric fields J. Chem. Phys. 111, 2556 (1999); 10.1063/1.479533 Autoionizing process of double rydberg states in atom AIP Conf. Proc. 388, 273 (1997); 10.1063/1.52193 Isotope shift measurement of autoionization Rydberg states of Sm AIP Conf. Proc. 388, 327 (1997); 10.1063/1.52169 Rydberg states near the ionization continuum: Autoionization in ammonia J. Chem. Phys. 101, 6523 (1994); 10.1063/1.468346

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termined from the absorption spectroscopy. No resid­ual signal was observable at 1 ms indicating that there are no Raman contributions in this region from other than the transient species.

The Raman band at 1647 cm-1 must be attributed to a short lived species with a moderately strong absorption at 438 nm, pointing very clearly, as indicated above, to the p-aminophenoxyl radical. By analogy with the similar frequency (1620 cm-1) observed for p-benzo­semiquinone radical anion, this band is almost certainly due to the ring stretching mode. It is seen in Fig. 1 that the bandwidth observed is -10 cm-1 broader than that for the semiquinone radical and slightly asym­metric, indicating the possible overlap of several bands. A NH2 bending vibration could be present in this region (a Raman band at 1621 cm-1 is assigned to NH2 in ani­line).8 A weak band is seen at 1434 cm-1• This fre­quency is identical to that of the CO stretch in p-benzo­semiquinone radical anion. 9 At the present the assign­ment of this band must be regarded as uncertain.

The ESR parameters of p-aminophenoxyl radical7

show that its electronic structure is more like that of p-benzosemiquinone than phenoxyl radical. In particu­lar, the drop ing factor from 2.00461 in phenoxy I radi­cal to 2.00377 in p-aminophenoxyl radical indicates that a considerable fraction of the spin density has been shifted to the nitrogen atom. This shift is apparently reflected in a 27 cm-1 increase in the frequency of the ring stretching mode. A shift to higher frequencies is somewhat surprising, particularly since the radical exists at the pH of this experiment in the protonated form rather than as the anion. One would like to have the Raman spectrum of phenoxy I radical for comparison but at the moment its absorption (398 nm) is inacces­sible to our experiment. While a Raman spectrum of phenoxy I radical has been reported to have been ob­served in stopped flow experiments10 it is clear that the concentration available on the ten ms time scale of such experiments « 10- 7 M) is insufficient for the reported observations. 11

We are continuing development of the methods de­scribed here and expect to examine the Raman spectrum

Autoionizing Rydberg states in Na2

S. Leutwyler, T. Heinis, and M. Jungen

of this radical in more detail. With improvements in beam targeting, laser intensity, the optical train, and signal proceSSing, it should be possible to achieve sen­sitivities one to two orders of magnitude greater. Such im­provements should also make it possible to carry out studies in the 10 to 100 ns time domain. Time resolu­tion is currently primarily limited by the 10 ns width of the nitrogen pump laser and the low sensitivity so that relatively long irradiation pulses are required. With incorporation of a YAG pumped dye laser system it is expected to be possible to examine essentially all spe­cies which absorb significantly at wavelengths longer than -250 nm.

We wish to thank Mr. Terrence Deal for his valuable assistance with this experiment.

a> The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2303 from the Notre Dame Radiation Laboratory.

lSee, for example, R. Wilbrandt and N. H. Jensen, J. Am. Chern. Soc. 103, 1036 (1981); R. Wilbrandt, N. H. Jensen, P. Pagsberg, A. H. Sillesen, and K. B. Hansen, Nature 276, 167 (1978); G. H. Atkinson and L. H. Dosser, J. Chern. Phys. 72, 2195 (1980); R. F. Dallinger, J. J. Guanci, W. H. Woodruff, and M. A. J. Rodgers, J. Am. Chern. Soc. 101, 1355 (1979).

2p. Pagsberg, R. Wilbrandt, K. B. Hansen, and K. V. Weis­berg, Chern. Phys. Lett. 39, 538 (1976).

3R • Wilbrandt, N. H. Jensen, P. Pagsberg, A. H. Sillesen, and R. E. Hester, Chern. Phys. Lett. 60, 315 (1979).

4p. C. Lee, K. Schmidt, S. Gordon, and D. Meisel, Chern. Phys. Lett. 80, 242 (1981).

sR. Wilbrandt, N. H. Jensen, P. Pagsberg, A. H. Sillesen, K. B. Hansen, and R. E. Hester, J. RamanSpectrosc. 11, 24 (1981).

6G. N. R. Tripathi and R. H. Schuler, J. Chern. Phys. 76, 2139 (1982).

1p. Neta and R. W. Fessenden, J. Phys. Chern. 78, 523 (1974).

aG. N. R. Tripathi, J. Chern. Phys. 73, 5521 (1980). 9G. N. R. Tripathi, J. Chern. Phys. 74, 6044 (1981). lOH. Shindo and J. Hiraishi, Chern. Phys. Lett. 80, 238

(1981). ltG. N. R. Tripathi and R. H. Schuler, Chern. Phys. Lett.

(in press).

Phusikalisch-Chemisches Institut, Universtiit Basel, Klingelbergstr. 80, 4056 Basel, Switzerland

H. -Po Hlirri and E. Schumacher

Institutf. anorgan. Chemie, Freiestr. 3. 3012 Bern. Switzerland (Received 28 December 1981; accepted 15 February 1982)

Alkali dimers can be resonantly two-photon ionized via selected excited rovibronic states using the narrow­band visible outputs of two independently tunable dye laser sources. 1 Such sequential two-photon ionization

studies have recently been carried out for the super­sonically cooled dimers N~, NaK, and ~ produced in pure alkali metal beams. 2,3 Adiabatic ionization poten­tials were determined by the sharp onset of photoion

J. Chern. Phys. 76(8). 15 Apr. 1982 0021 ·9606/82/084290·03$02.1 0 © 1982 American Institute of Physics

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19.70 18.57 17.58 16.61 15.62 14.55 13.62 12.60 11.63 y.7

~ 1 1 1 :1 1 :I 1 1 1

·c 19.72 16.58 15.60 14.57 13.62 12.62 y=6 :::I I, I: 1 :1 1 1 .D 25.63 23.60

19.66 16.61 ... y=5 26.67,24.60 22.61 21.61 20.62 18.61 17.61 15.62 14.57 13.62 y=5 2 1 1 I: 1 1 1 1 1 I: 1 1 I 1 :1 1

a I y=4 22.63 21.61 20.61 19.73 18.61 17.59 16.60 lS.61 y=4 C I 1 1 1 t 1 1 t 1 01 iii y=3 21.61 20.57 19.75 18.61 17.55 y=3 c I I I t I I .2

FIG. 1. Relative photoionization cross section of N~ pumped to the B Illu (Vi =7 ,J~= 1-7) states (R bandhead excitation). The strong autoionization lines are assigned as n.6 Rydberg series converging to the v = 3, 4,5,6,7 limits. Note that the continuum cross section is virtually nil at threshold and is only observable for energies ~39800 cm- I • Perturbations are indicated by verti­cal dashed lines for n* = 19.6, 16.6, and 14.6. The abscissa is given in cm- I (vacuum). The upper part of the figure shows the transmission pips from the Fabry-Perot etalon with FSR = 4. 906 cm-I (downwards) and the optogalvanically detected neon refer­ence lines (upward). Wavelengths are given as Aalr.

signal. In all three cases, extensive autoionization structure is observed and is usually the dominant con­tribution to the ionization cross section for the first 1000 cm-1 above threshold.

In the case of N~, the autoionization structure had proven quite resistant to analysis: the Na2" ion core vibronic levels are closely spaced (-120 cm-1), thus a number of different electronic Rydberg series, each in turn converging towards up to five different ion core vibrational states, may be observed. The autoioniza­tion structure was measured for Na2 pumped to the v' = 2-9 vibronic levels10 of the B 1IIu state. The pump laser encompasses the R(0)-R(6) transitions within its bandwidth of -0.5 cm-1• Partial analysis has now been achieved for the high v' = 7, 8 intermediate vibrations, whose structure is less complex than for the other vi­brations, as illustrated in Fig. 1. One prominent Ryd­berg series with effective quantum numbers n. 6 con­verging on v = 5 is clearly recognized. Four less in­tense n. 6 Rydberg series converging towards v = 3,4, 6,7 are also assigned and correlate well with autoion­ization structure observed via the next lower and higher intermediate vibrations v' = 6 and v' = 8. Analysis of the observed terms Tn could be performed using the simple Balmer-type formula Tn=T~ -Ry/(n-o)2 and a practically constant quantum defect 0 = O. 4 ± 0.03. These dominant series are accompanied (in Fig. 1 and also in many other autoionization spectra) by lines to lower energy that exhibit the same intensity patterns as the main series. The effective quantum numbers are approximately -n.43, but the line positions vary unsys­tematically and strong perturbations are inferred. Weak perturbations of the AV = ± 1 type can also be seen in Fig. 1: the terms (14. 6/v + 1), (16.6/v), and (19.6/ v - 1) are quasidegenerate and energy shifts as well as intensity exchanges are observed.

The electronic configuration of N~ (B 1IIu) is KKLL(3sa~)(3p1fu) and strong transitions to ns and nd singlet configurations are expected. The effective quan­tum numbers n* of the corresponding Rydberg states can be estimated by the frozen core model (see Ref. 4 for method and basis set). Thus we expect the follow­ing series:

- nsa~(1L;), n* = n. 0 ,

-ndag (1 L;) , n*=n.6,

- nd1fg (1 IIg ) , n* =n. 0 ,

-ndog (1 Ag) , n*=n.4.

So far, only two series have been observed; we pre­sume that the dominant n. 6 series is nda~, while the perturbed n. 45 series is ndo,. No evidence for n. 0 series has yet been found.

By extrapolation of the observed series to their (vi­brational) limits T ~(ii), we obtain the ion ground state vibrational terms 2L ;(ii=3-7), which are gathered in Table I. A quadratic fit to the observed terms yields two vibrational parameters we == 120. 6 ± 2.4 cm-1 and WeXe == 0.5 ± O. 2 cm-1• Schawlow et al. 5-7 have per-

TABLE I. Nai X 22::; vibrational term energies [referred to Na2 X 12::; (v" = 0, J"=O)].

v rl, (cm- I )

3 39836 ±2 4 39954 ±1 5 40069±1 6 40184±1 7 40297 ±2

J. Chern. Phys., Vol. 76, No.8, 15 April 1982

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formed an extensive study of Naz Rydberg states at lower principal quantum numbers (n = 3 -12) by two-step po­larization labeling spectroscopy. By extrapolation of the observed vibrational constant to n =~, they obtain we = 120. 3 cm-t, in excellent agreement with our value. Both values are also in fair agreement with previous theoretical calculations. 8,9

We can also determine the molecular ionization poten­tial by extrapolation to ii = 0 as I. P. = 394 81 ± 6 cm-1

•

The experimental error is fairly large, as two consecu­tive extrapolations are involved in the derivation of this value. Nevertheless, this spectroscopically determined value is Significantly higher (by 49 cm-1) than the pre­vious experimentally determined photoionization thresh­old. 3 This difference is attributed to field ionization, i. e., the lowering of the effective ionization potential due to the repeller field in the ion source. Contribu­tions by other ionization mechanisms such as blackbody radiation and collisional ionization are probably inSig­nificant; these questions are presently under experi­mental investigation.

We thank the SwiSS National Science Foundation (Pro­jects No.2. 883-0. 80 and No.2. 066-0. 81) and the CmA-Stiftung (Basel) for financial support.

IA. Herrmann, S. Leutwyler, E. Schumacher, and L. Woste, Helv. Chim. Acta 61, 453 (1978).

2S. Leutwyler, A. Herrmann, L. Waste, and E. Schumacher, Chern. Phys. 48, 253 (1980).

3S. Leutwyler, M. Hofmann, H. P. Harri, and E. Schumacher, Chern. Phys. Lett. 77, 257 (1981).

4M. Jungen, J. Chern. Phys. 74, 750 (1981). 5N. W. Carlson, F. V. Kowalski, R. E. Teets, andA. L.

Schawlow, Opt. Commun. 29, 302 (1979). 6N• W. Carlson, A. J. Taylor, and A. L. Schawlow, Phys.

Rev. Lett. 45, 18 (1980). 7N. W. Carlson, A. J. Taylor, K. M. Jones, and A. L.

Schawlow, Phys. Rev. A 24, 822 (1981). 8J • N. Bardsley, B. R. Junker, and D. W. Norcross, Chern.

Phys. Lett. 37, 502 (1976). ~. Kirby-Docken, C. J. Cerjan, and A. Dalgarno, Chern.

Phys. Lett. 40, 205 (1976). IOThe notation used for vibronic levels is: v' for N~ Blny

levels, v for Rydberg state levels, and v for N~ X2~; levels.

J. Chem. Phys., Vol. 76, No.8, 15 April 1982

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