LASER-INDUCED NEGATIVE IONIZATION : THE SF 6 + Ba SURFACE REACTION *

This paper reports on the results about the SF6 + Ba (surface) reactions as a function of the vibrational 
excitation of the polyatomic molecule. A thermal, vibrationally excited by a tunable CO2 laser, SF6 beam 
is collided with a Ba surface target under high vacuum conditions. The total negative ion yield is measured 
as a function of the laser wavelength. Preliminary results show a strong vibrational enhancement of the 
beam-surface reactions indicating important laser-assisted negative ionization effects. The results are 
discussed in the light of several possible reaction mechanisms.


INTRODUCTION
The importance of vibrational excitation in promoting gas-phase chemical reactions is well known in molecular reaction dynamics.In particular for endoergic reactions this vibrational selectivity has been proved to be a key feature of the so-called late barrier reactions.On the other hand, vibrational excitation is also very important in controlling chemical processes occuring in adsorbed layers.This is a consequence not only of the influence on the lifetime of a molecule on the surface after vibrational excitation, but also of the barrier reduction for chemisorbed molecules in the presence of laser radiation.
The present paper reports on preliminary results on the formation of negative molecular ions by selective surface ionization under the action of infrared Laser emission.4 The study is based on the well known fact that the yield of negative ions in the interaction of molecules with heated surfaces may be as high as 99% for some 201 hexafluorides.In addition the choice of SF molecule is also very convenient because several lines of a CO2 laser can be used to pump rovibratationally its v3 band.Thus the influence of vibrational excitation on the laser-assisted beam-surface ionization can be studied providing insight into the dynamics of the underlying physi and chemisorption processes.

EXPERIMENTAL
A schematic view of the laser-beam apparatus is displayed in Figure 1.Briefly an effusive SF6 beam impinges on a polycrystaline Ba surface.A tunable CO2 laser is used to excite the thermal SF6 beam as well as to heat the Ba surface.This is accomplished by focusing the laser on the surface collimated area at which the molecular beam collides.This procedure ensures not only a cleaner surface, but also a constant surface temperature while the laser wavelength is slowly tuned.
In front of the surface there is an ion collector to monitor the ion yield as the wavelength is changed.A typical run consists of measuring the ion yield by using a continuous SF beam excited by a modulated CO2 laser beam.For a given wavelength, the net signal consists of negative ion current with both the laser and " FL NzL Figure 1 Schematic view of the experimental set up.LC, CO2, Laser, LI-I, Heliun-Neon Laser.M, mirror.WM, Mixer mirror.FL, focal len.IW, infrared wall.MS, metal surface.WD, Ion collector.NO, nozzle oven.SK, Skimmer.beam on minus the background signal with the beam on and laser off.The signal properly amplified and filtered by a lock-in amplifier is then measured as a function of the CO2 laser wavelength.
Table I summarises the most relevant experimental conditions of the present work.Figure 2 shows a typical laser-induced negative ionization spectrum taken under the experimental conditions of Table I.It is interesting to point out the lack of positive ion current even with both the laser and beam on.In addition the good signal to noise ratio e.g.laser on beam on/beam off is noticeable and rules out the possible contribution of the thermoionic emission to the observed signal.The thin solid line represents the CO2 laser power as a function of the wavelength.The heavy solid line through the open circles corresponds to the measured negative ion current at the same experimental conditions.One of the most significant aspects of the present results is the non-ther- mal character of the observed phenomenon.Notice, for example, the absence of signal at v 980 cm in spite of the fact that the laser power, and likely the surface temperature, is about the same as that for v 945 cm-.The clear vibrational en- hancement of the ion yield, underlying this energy (laser wavelength) selectivity, is evident.  2 Negative ion signal (circles) and laser power as a function of laser wavenumber.--Laserpower (ordinate scale in arbitrary unit on the right).Negative ion signal in arbitrary units.The signal corresponds to the total negative ion signal collected as the SF beam impinges on the Ba surface.Notice the clear selectivity of the experimental data.i.e.No signal is observed beyond v > 960 cm-.Laser linearly polarized at 90 degrees with respect to the surface normal.

Possible reaction mechanisms
Unfortunately the negative ion yield spectrum was taken without mass selection.Therefore the total negative ion signal may well be due to several possible products: the fast SFg and the slow SF.These two ions can be produced by the following processes (i) fast, non-reactive negative ionization 1. SF + hv (IR CO2 laser) S66 2. S + Ba(s) S... Ba(s) --SFg + Ba(s) (Physisorption) (ii) slow, reactive negative ionization 1. SF -I-hv (IR CO2 laser) SF6 2. S + Ba(s) SF Ba(s) (Chemisorption) 2S + F2Ba(s) (3a) 3. 2S..Ba(s) 2S + F2 + Ba(s) (3b)  In principle, from a theoretical point of view, one could also consider another possibility: that is the negative gas-phase ionization.
(iii) SF*6 + Ba /" SFg6 + Ba SF5 + FBa induced by fast laser desorption of the Ba metal and multiphoton excitation of the polyatomic molecule.Such multiphoton excitation process is necessary since the ground state reactions represented by iii are endoergic or require a translational en- ergy threshold higher than the total kinetic energy available in the present experiment (ET 0.1 eV).The fact that we observe no positive signal when we reverse the collector bias (e.g. to collect positive ions) in addition to the lack of multiphoton processes (see below) rules out the mechanism outlined by equation (iii).
In view of the lack of the mass selection, as pointed out earlier, we are unable to distinguish which reaction pathway e.g. or ii is responsible for the observed laser- assisted negative ionization.We believe we may have both of them.
Comparison with other SF 6 spectra Another crucial aspect of the present work is to know whether multiphoton or single photon excitation is responsible for the beam-surface ionization process.
Figure 3 compares the present laser-assisted negative ion formation with: (a) the SF linear absorption cross-section (single-photon gas-phase absorption) from Ref. 8 and (b) the SF multiphoton dissociation rate as a function of the laser wavelength, from Ref. 9. It is interesting to point out the red shift of the present spectrum with respect to the linear uniphoton, gas-phase absorption cross-section of the SF 6.
In spite of the red-shift of the observed spectrum with respect to the single-photon absorption data, it is interesting to point out the close similarity between these two spectral shapes.On the other hand, the significant difference of the present data with respect to the multiphoton absorption spectrum rules out the role of the multiphoton processes.It appears that the observed laser-assisted negative ionization is a single- photon process which red-shift might be due to the softening of the intermolecular potential of the adsorbed SF 6.
Work is now in progress to unravel the molecular mechanism of this laser-assisted surface ionization by means of time-of-flight analysis of the reaction products.

Table 1
Experimental Conditions Laser-induced negative ionization Table II lists the ion current intensities observed in a typical run.

Table 2
Typical Ion Current Intensities