Collinear photodetachment spectroscopy

NOMB

Nuclear Instruments and Methods in Physics Research B79 (1993) 159-161 North-Holland

Collinear photodetachment

spectroscopy

Beam Interactions with Materials 8 Atoms

*

D. Hanstorp, M. Gustafsson, U. Berzinsh ’ and U. Ljungblad Department of Physics, Chalmers University of Technology and Universiiy of Gdteborg, S-41296 G&eborg, Sweden

A new ion beam apparatus, designed for collinear photodetachment experiments, has been constructed. Negative ions are formed on a surface ionizer, mass selected in a sector magnet and directed into a field-free region where they are merged with a laser beam along a 50 cm long path. A new secondary electron emission detector has been constructed in order to detect neutral atoms formed in the photodetachment process. The design of the detector makes it possible to use pulsed lasers which, in combination with a gated detection, gives a very good signal-to-background ratio. In a first experiment, the electron affinity of atomic iodine has been determined to be 3.059038(10) eV. The onset of the photodetachment process was observed when the

wavelength of the laser light was scanned, and the result was fitted to the Wigner law in order to determine the photodetachment threshold.

1. Introduction

A newly constructed collinear laser-ion-beam apparatus, especially designed to study the photodetachment process, is described. The photodetachment process, which corresponds to the photoionization process for neutral atoms, is normally the only way to study negative ions with optical methods since most negative ions only have one bound state. Using the laser photodetachment threshold method [l], by which the onset of the photodetachment process is studied, the electron affinities of atoms can be determined with very high accuracy. The technique with collinear laser and ion beams is especially favorable when the threshold of the photodetachment process is studied, since it gives the highest optical resolution. Furthermore, the long interaction path is of great importance since the photodetachment process is fairly weak, especially near the threshold.

quadrupoles placed 50 cm apart with their axis vertical to the plane of deflection. The number of ions that has passed through the two quadrupole deflectors is moni-

Ultra High Vacuum chambl

Detector box

__

2. Experimental setup Negative ions are formed on a hot LaB, surface in a surface ionization process. The ions are accelerated to 3 keV and mass analyzed in a sector magnet with a radius of 50 cm. The ions then enter the analyzing chamber, shown schematically in fig. 1, where they are deflected by 90” in each of two electrostatic * This work has been supported by the Swedish Natural Science Research Council (NFR) and by the Swedish Institute. 1 Permanent address: Department of Spectroscopy, University of Latvia, 19 Rainis Blvd, Riga 226098, Latvia. 0168-583X/93/$06.00

NC.UUd particles

-

Faraday CUP / Electrostatic deflectors

\

I

Interaction region

aReflected asxx bean

M&S

analyzed ion beam

Fig. 1. Schematic of the analyzing chamber.

0 1993 - Elsevier Science Publishers B.V. All rights reserved

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D. Hanstorp et al. / Collinearphotodetachment spectroscopy

tored with a Faraday cup placed after the second quadrupole. The ion current measured at this point is typically a few nA. The ions are illuminated with laser light in the region between the two quadrupoles. This light can be directed either parallel or antiparallel with respect to the ion beam direction. Fast atoms produced in the photodetachment process are not affected by the second quadrupole. Instead, they impinge on a glass plate coated with tin doped indium oxide placed behind the second quadrupole. Secondary electrons produced by the atoms are detected with a channel electron multiplier (CEM) operated in a pulse counting mode. A serious problem in the neutral beam detector is introduced when pulsed laser light is used to induce the photodetachment process. The intense laser radiation produces electrons in the neutral detector glass plate by the photoelectric effect. This signal can be so intense that it strongly saturates the CEM, resulting in a recovery time of many microseconds. This problem has been solved by blocking the CEM during the laser pulse. The laser pulse strikes the glass plate some time before the neutral particles due to the travelling time of the atoms from the interaction region between the quadrupoles to the glass plate. A negatively biased fine mesh grid is placed in front of the CEM. The potential applied to this grid controls the passage of electrons from the glass plate to the CEM. These electrons are blocked during the laser pulse and are transmitted when the corresponding neutral atoms impinge on the glass plate [2].

3. Resolution and signal levels The resolution obtained in a laser-ion-beam experiment is determined by a combination of the laser bandwidth, the properties of the ion-beam apparatus and the signal to noise ratio. The frequency width dependence on the ion-beam apparatus properties can be divided into three parts. First, the energy spread of the ions causes a frequency broadening. By using the collinear geometry, however, this broadening is substantially reduced. The initial energy spread of the ions, due to the nonzero temperature in the ion source, is compressed when the ions are accelerated by the ion source potential [3]. Second, the non-zero divergences of the laser and ion beams cause broadening. The laser beam can be made almost parallel, whereas the divergence of the ion beam is determined by two apertures defining the interaction volume. Finally, the limited interaction time causes a broadening due to the Heisenberg urrcertainty relation. When cw lasers are used, the interaction time is determined by the transit time for an ion through the interaction region, while in pulsed laser experiments it is determined by the pulse length.

As an example, we estimate the spectral resolution obtained with our apparatus, assuming a beam of 3 keV Cl- ions illuminated with 20 ns long pulses of 343 nm laser light, to be 45 MHz. The energy spread of the ions contributes with 34 MHz, the nonzero divergence of the laser and ion beams with 7 MHz, and the limited interaction time with 4 MHz [4]. This value should be compared with the bandwidth of a cw single mode dye laser of typically 1 MHz, and the best pulsed dye lasers with bandwidths of around 1 GHz. Clearly, when cw lasers are used, the resolution attainable in an experiment is determined by the properties of the apparatus, while in pulsed laser experiments the resolution is set by the bandwidth of the laser. In the choice between pulsed and cw lasers one also has to consider the signal levels obtainable in the two cases. A negative ion traversing the interaction region has an estimated probability for collisional detachment of 2 x 10m6 (p = 10m9 mbar, gcon.= lo-l5 cm21. For a cw laser experiment (I,,,,, = 5 W/cm’, uPhotod,= lo-‘s cm2) the probability for photodetachment is 4 X lop5 resulting in a signal-to-background ratio of about 20 : 1. The signal level is improved significantly by using pulsed laser excitation instead. The probability for the photodetachment of an ion interacting with pulsed laser light is about 4% (I,,,,, = lo6 W/cm2). Selecting only the ions that are in the interaction region during the laser pulse, by use of a gated detection technique, yields a signal-to-background ratio of about 20000 : 1 [4]. Consequently, the signal-to-background ratio in this example is some three orders of magnitude better if a pulsed laser is used. This fact can partly compensate for the loss of resolution due to the large bandwidth of the pulsed laser.

4. Measurements

and results

The electron affinity of iodine and chlorine have been measured using a pulsed dye laser equipped with an intracavity etalon (bandwidth = 1.2 GHz). The laser wavelength was scanned over the photodetachment threshold with the laser and ion beams directed both parallel and antiparallel. By taking the geometrical mean of the two measured threshold wavelengths, the Doppler shift can be eliminated to all orders, resulting in the unshifted threshold wavelength [4]. Fig. 2 shows the number of iodine atoms produced by the photodetachment process plotted versus the laser wavelength. The direction of the laser-beam was reversed at A = 405.31 nm. The solid line in the expanded part of the figure is a least square fit of the data to the Wigner law [S] where the small humps are due to the hyperfine structure (HFS) of the groundstate of the iodine atom [6]. The nonzero signal below the threshold was caused by the so called amplified spontaneous emission CASE).

D. Hanstorp et al. / Collinear photodetachment spectroscopy

161

Using the technique described above, the electron affinities of iodine and chlorine have been determined to EA[I] = 3.059038(10) eV and EA[Cl] = 3.61276@) eV, in good agreement with previous experiments [8,9]. The wavelength scale was determined by means of a Fabry-Perot reference etalon and well known atomic lines situated in the vicinity of the thresholds [6]. The error in the iodine experiment of 2.5 GI-Iz was mainly due to statistical uncertainties in the threshold determinations (1.4 GHz) caused by the laser bandwidth of 1.2 GHz. In the chlorine experiment the uncertainty of the reference lines set the limit of the accuracy. 405.2

405.5

405.4

405.3

Wavelength

(nm)

Fig. 2. Photodetachment signal of I- vs laser wavelength. solid line is a fit to the Wigner law.

The

broadband radiation at the emission peak of the laser dye is always produced in pulsed dye lasers. In fig. 3, which shows the photodetachment threshold of chlorine at 343 nm, the signal due to the ASE has almost been eliminated. This was accomplished by using a laser dye (BMQ [7]) that has its emission peak (357 nm) at a wavelength much longer than the threshold wavelength. With this dye the photon energy of the ASE is too low to induce the photodetachment process. The HFS in chlorine is much smaller than in iodine and cannot be seen in this recording. Instead, the laser bandwidth of 8 GI-Iz causes a small deviation from the fit to the Wigner law in the vicinity of the threshold. (In this recording the laser was operated without the intracavity etalon.) This

5. Conclusions We have shown that pulsed lasers can be used for collinear photodetachment experiments. This has been made possible by the construction of a new type of neutral particle detector. Using a gated detection, a very good signal-to-background ratio can be obtained that partly compensates for the large bandwidth of pulsed dye lasers. It also is demonstrated that the amplified spontaneous emission background can be strongly reduced by choosing a laser dye with its emission maximum at a wavelength longer than the threshold wavelength. Finally, it has been shown that threshold determinations with statistical uncertainties of a few GHz can be obtained. This is of the order of the laser bandwidth, and for a significant improvement of this resolution one has to use a laser with a smaller bandwidth.

References 111H. Hotop and WC. Lineberger, J. Phys. Chem. Ref. Data

343.02

343.04 343.03 Wavelength (nm)

343.05

Fig. 3. Photodetachment signal of Cl- vs laser wavelength recorded with parallel laser and ions beams.

14 (1985) 731. D. Hanstorp, Meas. Sci. Technol. 3 (1992) 523. S.L. Kaufman, Opt. Comm. 17 (1976) 309. D. Hanstorp. Thesis at the University of Giiteborg (1992). E.P. Wigner, Phys. Rev. 73 (1948) 1002. D. Hanstorp and M. Gustafsson, J. Phys. B25 (1992) 1773. Lamdachrome Laser Dyes, Lamda Physik, Giittingen, Germany. [8] CR. Webster, IS. McDermid and C.T. Rettner, J. Chem. Phys. 78 (1983) 646. 191 R. Trainham, G.D. Fletcher and D.J. Larson, J. Phys. B2O

[2] [3] [4] [S] [6] [7]

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