Photoionization of acetylene near threshold

49 International Journal of Mass Spectrometry and Ion Physics, 11 (1973) 49-56 Q Ekevier ScientiSc Publishing Company, Amsterdam - Printed in The Ne,~erlands

PHOTOIONIZATION

VERNON

H.

OF ACETYLENE

DIBELEKAND

institute for Materials

JAMES

NEAR

THRESHOLD

A. WALKER

Researcir, National Bureau of Standards,

Washington,

D.C. 20234 (U.S.A.)

(Received 13 September 1972)

ABSTRACT

Photoion yield curves for C2H2 and C2Dz cooled to 118 K are remeasured at 0.2 A intervals from ionization threshold to about 0.6 eV above threshold. Clearly defined autoionization features are observed superimposed on vibrational step structure. The latter observation suggests molecular predissociation occurring in competition with the autoionization process. A simple means of estimating Franck-Condon factors for the direct ionization in the presence of autoionization gives values in agreement with photoelectron spectroscopy data. Comparisons of observed and calculated Rydberg levels converging to vibrationally excited states of the molecular ion are discussed briefly.

INTRODUCTiON

Photoion yield curves for the molecular ionization of acetylene have been obtained by several workers by using a combined vacuum ultraviolet monochromator and mass spectrometer’ - 5. First ionization threshold values are in excellent agreement with data obtained by photoelectron spectroseopy6, by vacuum ultraviolet absorption spectroscopy’* ‘, and by recent monoener@ic electron impactg. However, some discrepancies have arisen in the interpretation of curve shape near threshold. Early studiesls ’ of ‘C&I, and C,D, suggested a simple step structure corresponding to the zero&, first, and second vibrationally excited states of the ion. On that basis, theoretical calculations of Franck-Condon factors, assuming certain models, were compared favorably with the reported curves. The calculations were also shownlO to agree well with_ photoelectron spectroscopy data6. In subsequent studies, however, Brehm4.and Omura et aZi5 reported weak structure superimposed on the steps and although their data showed considerable scatter, the structure was interpreted as evidence for Butoioniz&.ion~.

50 In the event that autoionization is an important contributor to the ion yield curve, comparisons of calculated and observed Franck-Condon factors based on photoionization could be of doubtful validity. Structure due to autoionization has been observed in ion yield curves of a number of diatomic and triatomic molecules’ r. However, little convincing evidence has been presented for the occurrence in larger polyatomic molecules; e.g. hydrocarbons 12*13_ As we have made some significant changes in the early photoionization mass spectrometer resulting in improved ion intensity, energy resoluticn, reliability of photon intensity measurements and the ability to cool the ion source, we were prompted to again study the photoionization of acetylene and to present a brief report on the results for the molecular ionization_ A more detailed account including dissociative ionization will appear at a later time.

EXPERIMENTAL

The basic arrangement of monochromator and mass spectrometer is unchanged from that previously described l4 . In brief, however, the initial ion source was replaced by an axially-symmetric immersion lens ion source* followed by a d.c. quadrupole lens’ 5. A stepping motor with pre-set indexer and drive is coupled to the lead screw of the grating mount. For the 1200 groove mm-r grating, one step is equivalent to a 0.025 A increment in wavelength at the exit slitAs the astigmatic image arriving at the exit slit of the monochromator is not a straight line, a suitably curved 50 pm slit16 was substituted for the original exit slit. The photon detector is essentially unchanged. However, a current digitizer and scaler are added with suitable master controls for both ion and photon counting.

RESULTS AND DISCUSSION

As a test of the capability of the apparatus, the ion yield curve for Ar+ was obtained with an optical resolution of 0.5 A from 7Y0 to 776 A using the Hopfield continuum of helium as the .bhoton source. The unsmoothed results of a typical run are shown in Fig. 1. Data points were taken at 0.1 _&intervals and the error bars indicate the estimated uncertainty in a single obserration. Following the initial onset, the well-known autoionization of Rydberg levels is observed above the ‘P+ limit converging to the ‘P+ limit. The autoionization apparently joins the continuum smoothly through the ‘_P+ threshold with no * Details of this simple lens, designed by K. E. McCulloh expressly for the photoionization mass spectrometer, will be published.

15.79 I

15.69

I

UUERGY, eV 15.89 I

16.00 I

i

Fig. 1. Photoion yield curve for argon obtaiswd with an optical resolution of 0.5 A. The arrows locate the known spectroscopic limits for the ?Pg and 2Ps states of the ion. The error bars indicate the estimated uncertainty in a single observation of the ion yield.

indication of a new onset, This is as expected from theory” and has been observed previously by Chupka and Berkowitz’ 8. Furthermore, the locarion of the spectroscopic limit lgof 12?109_9cm- 1 (786.72 I%) for the ‘P, state is shown by arrow in

the figure as occurring

at approximately

mid-point

of the distance

between

the

base line and the deepest minirnu,m following the first autoionization peak, as might be expected from a step function for direct ionization and the slit function

for the present experiment.

It is important

to note here, however,

that the relative

cross-sections for ‘P+ and ‘P* states are not obtainable from the figure. Figure 2 shows the photoion yield curve for CZHZ+ obtained from 1095 to 100 A under experimental conditions similar to those for argon except for an ion

source temperature of 1 Ifs K. The continuous emission spectrum of argon *was used as a photon source for the wavelength region 1095 to 1070 A. The manylined hydrogen spectrum was used for the region 1090 to 1030 ,& with a very satisfactory reproducibility

of data in the ovedapping

region.

52

. . . . . . . . . . i____-1033

ENERGY, eV Il.59

Il.48 I

Il.37 I

i-

I IOrn

I

I!.?0

I I I 1070 1060 WAVELENGTH, I%

II.81

I

I lo50

II.92

I IO’40

Fig. 2. Photoion yield curve for &Hz obtained with the ion source cooled to 118 K. The manner ofestimating Franck-Condon factors for direct ionization is indicated. CaIcuIated Rydberg levels are located for severai series converging to vibrationally excited states of the ion.

An important difference between previously published curves and the present work is immediately apparent. Although the curve rises sharply beginning at about 1089 A, a well-defined maximum is obserx;ed at 1086 A. Thereafter, the curve contains a series of peaks diminishing both in intensity and in definition to an apparently featureless region at 1070 81. These features indicate weak autoionization of one or more Rydberg series ;uperimposed on a direct ionization continuum and converging to the several limits of the tibrationalIy excited states of CzH2+. The spacing of the steps is consistent witb the v2 stretching frequency of the ion as previously reportedly 6. Unlike the case of argon, the exact threshold is difFicult to locate due to the rotational envelope of approximately 0.01 eV at 118 K. However, on the basis of the slit function shown for argon, it is taken to be that point on the rapidly rising yield curve that is midway between the base line and the deepest minimum in the autoionization structure (about 1084 A). The resulting value of 1087.7+0.5 A (11.398 4 0.005 eV) is in good agreement with previously reported values. However, the given uncertainty is intended to indicate that the energy dieerence between the rotationless molecule and the rotationless ions lies within this interval. It is interesting to note that the oscillator strength density does not vary smoothly through the threshold of the higher state as demonstrated

53 for argon. Thus, photon absorption is not leading solely to ionization and the Rydberg states converging to the several vibrational limits are being depopulated by a process in competition with the autoionization process. As previously noted18, a very probable one in the case of molecules is predissociation to neutral fragments. The apparent gradual rise in absorption cross-section between the steps may be an ex@erimental artifact restthing from. the finite resolution of the monochromator. The possibility of calculating Franck-Condon‘ factors for the direct ionization process is of considerable interest. The difficulty arises in estimating that portion of the ion yield curve due to direct ionization. Baker and Turner6 have published a curve for the first band of the photoelectron spectrum of acetylene. By scaling their figure, we estimate the relative intensities of the 0,0, 0,l and 0,2 transitions to 1.0, 0.33, and 0.11, respectively_ These are shown as vertical arrows on the arbitrary scale of our Fi g. 2. Thus, Franck-Condon factors in agreement with the photoelectron spectroscopy data are obtained in the present work if we adopt the empirical choice of absorption cross-section for direct ionization to be that from base line to autoionization minimum. Note that this would -beconsistent with our manner of selecting the threshold for the ‘I’+ state of Ar+ as shown in Fig. 1. Further support of this point may be obtained from the photoion yield curve of did=uteroacetylene shown in Fig. 3. The curve, normalized to equal the ion yield of &HZ+ at 1070 A, shows a similar shape and structure. The threshold

. . .. .. . .

35

1

I065

I

1075

I

I

iO65 WAVELENGTH, &

I

1Ok5

1045

Fig. 3. Photoion yield curve for CzD2 obtained in the same manner as for Cll-12.

54 value of 11.404+ 0.005 eV is greater than that of C,H, by an amount consistent with vibrational zero-point energy differences3. Platzman2’ has argued that ionization of polyatomic molecules occurs to a large extent through autoionization and predicted an isotope effect in the ionization efficiency. The latter results from the assumption that the autoionization process itself is unaffected by isotopic substitution. However, it is in competition with other processes (e-g- pre-dissociation as ‘noted above) that are slower in the deutero-molecule than in the ordinary molecule. The prediction has been confirmed for molecular ionization produced by various particles, -including photons”. A comparison of the normalized curves in Figs. 2 and 3 shows increased intensity of autoionization relative to direct ionization for C,D,. Nevertheless, the expected agreement of Franck-Condon factors for C2H2+ and C,D,+ results when direct ionization yield is estimated as indicated above. Two Rydberg series converging to approximately the same ionization limit of acetylene were first identified by Price’ and later confirmed by Wilkinson’. According to Price, the series were represented by the relations v, = 92076-_(n-0.50)-2,

series I,

and v, = 91950-

R(n-0.95)-2,

series II,

respectively, the difference in limits possibly representing the splitting of the lowest 211 state of the ion. Using the same S values and assuming the present threshold and average vibrational interval of 0.227 eV for the ion6, we have calculated the members of two Rydberg series converging to each of three vibrational states of C21-12+ as indicated by the solid lines above the curve in Fig. 2. The estimated uncertainty in frequency for any level is of the order of 80 cm-‘. The calculated series II members correlate well with observed peaks, particularly in the first vibrational series. The series I members, however, are apparently too weak to be observed or are unresolved in the valleys. At least equally good correlation with observed peaks is obtained if members are calculated using 6 = 0.35 in the Rydberg formula as indicated by the dashed vertical lines. However, it is important to note here that the present limitation in resolution (0.5 A) makes difficult the unequivocal identification of these series. Although the value of6 = 0.50 is generally considered large for a d atomic series, Greene et al.22 have recently presented further confirmation of the correct identification of both series. Therefore, although no previous study has so indicated, our results within the above limitation could be interpreted as evidence for the existence in the acetylene vacuum ultraviolet spectrum. We have also attempted to correlate autoionization peaks in yield curve with members of Rydberg series calculated in the same CzH2, except for the use of an average vibrational interval of 0.21

of three series the C2Dt+ ion manner as for eV. The latter

55

is somewhat greater than determined by Baker and Turner but results in better agreement with the presently observed onsets of vibrational states of the ion and with the value calculable from the C2HZ9 interval. As can be seen from the figure, generally good correlation is obtained for series II and III as in thecase of &HZ. In addition, however, the increased relative intensity of autoionization apparently permits observation of some members of series I (6 = OSO), particularly at wavelengths of 1086 and 1082.5 A for the series converging to C2DIC (0 =.j:1) and at -.. 1067.5 A for CzD2+ (Y = 2). Duncan’ 3 has summarized conclusions concerning Rydberg series observed in the vacuum ultraviolet spectra of acetylene and substituted acetylenes. The comparison of work on the latter molecules by several investigators indicates possible confusion in series and threshold identification, especially for propyne. However, there is evidence in both propyne and butyne for three Rydbergseries having quantum defects of approximately 0.95, 0.53, and 0.33, respectively. The assumption that similar series would be observed in acetylene supports the suggestion of a third series in that molecule.

ACKNOWLEDGEMENTS

We are indebted to Dr. K. E, McCulloh for supplying the details of design of his ion source and for assistance in other modifications of the apparatus; and to Dr. H. M. Rosenstock for frequent, helpful discussions concerning the interpretation of the experimental results.

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