Volume 100. number 6 CHEMICAL PHYSICS LETTERS 30 September 1983

PHOTODETACHMENT THRESHOLD OF CN- BY LASER OPTOGALVANK SPECTROSCOPY

Robert KLEIN *, Richard P. MCGINNIS ** and Stephen R. LEONE i- Joint Institute for Laboratory Astrophysics. Arational Bureau of Standards and University of Colorado and Department of Chemistry, University of Colorado. Boulder. Colorado 80309, USA

Received 15 August 1983

Laser optogalvanic spectroscopy is used for the first time to obtain the photodetachment threshold for a molecular nqa- tivc ion, CN-_ The electron affinity for CN is determined to be 3.83-l f 0.004 cV.

l_ Introduction

In recent years numerous works on laser optogal- vanic (LOG) spectroscopy have been published [l] _ The laser optogalvanic effect is based upon the changes in discharge current resulting from modifications of the charge carriers when laser radiation is absorbed. There are two features that make this new technique an at- tractive alternative to more conventional spectroscopic detection methods: the method combines both simplic- ity and high sensitivity, and many unstable species such as radicals and ions can be generated in the dis- charSe itself.

Resonant transitions in atomic as well as molecular neutrals and ions have now been investigated by LOG spectroscopy. Recently, Webster et al_ [2] reported the application of LOG spectroscopy to determine an ac- curate value of the electron affinity (EA) of the I- atomic negative ion_ Walkup et al. [3] observed the first LOG spectra of molecular ions, Nt(B-X) and CO+(A-X). Here we report the first LOG detection of the threshold photodetachment of a molecular nega- tive ion, CN- _ Moreover, this is the first direct optical determination of the electron affinity of the CN radical.

* Present address: Fachbereich Chemie, Universitat Frankfurt, D-6000 Frankfurt. West Germany.

** Research Fellowship for Teachers, Joint Institute for Laboratory Astrophysics, 1982-83. Present address: Tougaloo, Mississippi 39174, USA.

* Staff Member, Quantum Physics Division, National Bureau of Standards.

The only other reliable determination of the EA is from the photoionization study on HCN by Berkowitz et al. [4], which provides a value of 3.82 2 0.02 eV directly from the difference in thresholds for the H+ + CN + e and H+ + CN- channels.

Much theoretical work has been done to obtain the ground state and excited state molecular properties of CN-, as well as the electron affinity of CN [5-7]_ The EA is defined as the energy separation between the low- est state of the anion and the lowest state of the corre- sponding neutral. The theoretical spectroscopic data of CN- in ref. [7], together with the known spectroscopic constants of CN [s] , are used in the extraction of the EA from the observed threshold energy. The basis for determination of the threshold ener,7 by optogalvanic spectroscopy lies in the fact that irradiation of the di- charge cell with laser frequencies higher than the de- tachment energy of the anion leads to electron photo- detachment and hence a fast and 1arS.e increase in dis- charge current [2] _ Since cyanogn halides are known to give good yields of CN- by electron impact [9], BrCN is found to be the precursor of choice for these experiments. Pulsed laser irradiation along with time- gated detection is a useful way to select out the signal due to the CN- detachment.

2. Experimental

The experimental apparatus is shown schematically in fig. 1 _ The optogalvanic cell is made of glass, 36 cm

0 009-2614/83/0000-0000/S 03.00 0 1953 North-Holland 475

Volume 100. number 6 CHEMICAL PHYSICS LETTERS 30 September 1983

DYE LASER

II POWERMETER

Fig-. I _ Schcnntic dralxing of the laser optoslvanic apparatus.

long and 3 cm internal diameter. and is specifically designed for flowing gases. The two hollow molybde-

num electrodes are separated by 2 cm and connected

to the dc power supply through a 50 l&2 ballast resis-

tor_ Two quartz windows mounted to the sides of the

cell allow transverse laser probing of the discharge

region. A pulsed preamplifier is coupled to the dis-

charge through a 0.01 PF capacitor. and the amplified

output is connected to a boxcar integrator_ The time

window of the boscar is set to optimize detection of

the CN- detachment signal. Pulsed laser radiation is

provided by a Nd : YAG pumped dye laser, operating

in the 320-330 nm region by frequency doubling the

DCIM dye. The ultraviolet laser output of ~1 mJ per

pulse is monitored with a power meter, which is con-

nected to a second channel of the boxcar averager in

order to normalize the optogalvanic signals for the

average laser power. The known me&table transitions

of the helium buffer gas give rise to large LOG signals

which serve as convenient internal wavelength calibra-

tion markers.

The CN- precursors are contained in a reservoir tube and can be swept through the discharge region by

a flow of helium. A variety of discharge conditions

were tried with BrCN. HCN, and CH,CN as precursors, either pure or in helium and either flowing or static.

Only BrCN gave CN- detachment signals, and the best

results were achieved by flowing helium at 270 Pa

(1 Pa = 7.5 X 1 O-3 Torr) over BrCN (v-p_ 27 Pa) at a

total flow rate of 5 X 1018 molecules s--l _ Quiet dis-

charges are typically achieved with 300-500 V. In

order to maintain a high degree of discharge stability,

the gas flow is carefully regulated with fine metering

valves on the inlet and outlet of the optogalvanic cell. The best stability was always achieved after freshly

cleaning the cell and electrodes. The lack of success in

detecting CN- photodetachment with HCN and

CH,CN is attributed to poorer electron impact yields

of CN- and to the deposition of carbonaceous films

which cause discharge instabilities.

3. Results and discussion

When the discharge of BrCN and He is irradiated at

320-330 MI, a strong optogalvanic signal is observed_ The signals indicate an increase in the discharge cur-

rent, and are rapid, typically 20-60 fl depending on the discharge characteristics. Such signals are typical of electron detachment processes, which result in imme-

diate increases in the discharge current when the highly

mobile electrons are released_ The electron detachment signals occur continuously as the laser is tuned through-

out a wide range of wavelen,@hs both above and below

the expected CN- detachment threshold at 324 run.

With BrCN as the precursor, both Br- and CN- ions

can be formed in the discharge_ The signal occurring to the long wavelength side of the expected CN-

threshold is attributed to detachment of Br-. When

the laser is tuned to 400 nm, well below the 363 nm

threshold for Br- detachment, no optogalvanic detach- ment signal is observed_

The CN- photodetachment threshold is observed

as an abrupt increase in the optogalvanic signal on top

of the continuous Br- background signal. The increase

is also accompanied by a slight shift in the time delay

of the maximum in the current change, indicative of a

new process opening up_ The photodetachment spec-

trum in the region of the CN- threshold is shown in

fig. 2. Typical settings of the boxcar time delay and gate window are 10 w and 10 J.E, respectively. The

sharp rise at 324.4 nm is attributed to the process

CN-(X’Z,u=O) I” --+CN(X2~,u=O)+e.

Rotational fine structure, however, has not been re-

solved_ The resolution of the experiment would have allowed us to observe vibronic transitions, for example

(0” = 1) + (u’ = 1) at ~325 mn [7], but no evidence

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Volume 100, number 6 CHEMICAL PHYSICS LETTERS’ 30 September 1983

’ 32582nm

Cl%- DETACHMENT

C WAVELENGTH

Fig. 2. Laser optoglwnic spectrum in the resion of the CN- photodetachment threshold. The strong peaks arc helium met stable transitions which serve as convenient internal markers

for the wavelen$h calibration.

for transitions other than (u” = 0) + (u’ = 0) has been

observed_ The abrupt photodetachment threshold, followed

by a steady increase in the detachment signal, has been measured in six scans under a variety of different dis- charge conditions_ The best signal-to-noise is achieved by pointing the laser beam into the Faraday dark space near the anode. The slight decrease in the baseline before the abrupt rise in fig. 2 is due to instability in the discharge current, and is not observed in most of the other runs. Two helium metastable transitions at 325.82 and 323.13 run, which result in enormous optogalvanic signals, provide an accurate wavelength calibration for the threshold in fig. 2. With this calibra- tion, the inflection point of the photodetachment is determined to be 324.44 * 0.03 ML

The analysis of a threshold photodetachment signal which lacks rotational resolution to obtain the desired electron affinity has been described in detail for the diatomic molecule OH- [lo] _ A key feature is that the threshold photodetachment law, CJ 0: E W, leads to a very sharp onset. The case for CN- is an analo- gousE1/2 thresholdlaw [11,12] andinthiscase,as will be noted below, it is even simpler to relate the ob- served threshold to the EA. Unlike resonant rovibronic transitions between two Z states, in electron detach- ment processes a Q-branch and even 0- and S-branches can occur. The lack of information on the relative in- tensities of these various branches does not allow an

accurate calculation of the rotational envelope_ How- ever, from the theory of Buckingham et al. 1131, it can be shown that the intensities of the branches decrease with increasing magnitude of AJ. More importantly, as shown for OH- by Hotop et al. [lo], the steep threshold region in OH- is characterized predominant- ly by a “piling up” of the nearly degenerate Q-branch transitions. Thus the observed threshold inflection point is determined predominantly by wavelengths of the Q- branch transitions arising from the states of maximum population in the rotational Boltzmann distribution_

In CN- and CN, unliie OH- and OH, the lowest allowed rotational levels that define the EA are J = 0 and K = 0, respectively_ Thus, only a small correction

is necessary to derive the EA from the threshold de- tachment ener,v, due to the slight change in energy for the J = K transitions at the maximum of the rotational population distribution_ Also fortuitously in CN- and CN, the near equality of the rotational constants in the ion and the neutral make this correction very small indeed_ Values of Be used in our analysis are l-88 cm-l [7] and 190 cm-1 [8] for the ion and the neutral, respectively.

For a 300 K rotational distribution, J,,,, = 7, and the EA is 3.821 eV. For a Jmds of 40, corresponding to an unreasonably high rotational temperature of 8000 K, the EA would decrease only slightly to 3.8 18 eV_ Since nothing is known about the rotational tem- perature of CN- in the discharge, but the recent LOG study of s [3] showed a rotational temperature of 840 K, a value of the EA corresponding to this slightly higher rotational temperature seems most reasonable. From the uncertainty in the inflection point and these uncertainties caused by the manifold of rotational states, we assign a value of 3.821 + 0.004 eV to the EA of CN. This value is in excellent agreement with the previous value obtained by Berkowitz et al_ [4] _

In conclusion, we have demonstrated that valuable information on molecular negative ions can be ob- tained with a relatively simple laser optogalvanic ap- paratus_ All of the ion experiments performed to date with the LOG spectroscopic methods have produced extremely high signal-to-noise on readily achievable densities of ions. Considering the problems associated with the use of conventional electrodes that are serii- tive to chemical reactions, it seems worthwhile to per- form such experiments with electrodeless discharges as well. Experiments with improved techniques might

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Volume 100. number 6 CHEMICAL PHYSICS LETTERS 30 September 1983

achieve rotational resolution and provide other details

about ion and detachment spectroscopy.

Acknowledgement

The authors gratefully acknowledge the support of the National Science Foundation and many stimulat- ing discussions with W-C. Lineberger. R-D. hIead and

K. Lykke.

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