THE QUENCHING OF EXCITED METAL ATOMS

PART l.--THE QUENCHING OF EXCITED THALLIUM ATOMS PRODUCED BY THE PHOTOLYSIS OF THALLOUS IODIDE VAPOUR

USING AN A.C. SPARK SOURCE

BY D. J. DOWLING AND E. WARHURST Dept. of Chemistry, The University, Manchester

Received 10th November, 1958

The quenching of excited thallium atoms produced by the photodissociation of thallous iodide vapour has been investigated. An a.c. spark between rotating zinc discs was used as the source of u.-v. light. The quenching effect of various gases was studied by ob- servations on the green line (5350A) of the atomic fluorescence of thallium. A photo- electric null method was used for the intensity measurements. The quenching cross- sections for various ethers, alcohols, hydrocarbons, oxygen and argon have been deter- mined.

One method for studying the quenching of excited atoms is based on the dis- covery by Terenin 1 that the photodissociation of a number of metal halide vapours by u.-v. light gives electronically excited metal atoms. This method possesses several advantages compared with that based on the production of excited atoms by absorption of resonance radiation by the metal vapour. These are, that radiation imprisonment and Lorentz broadening are negligible and that the quenching effect of gases which react chemically with the metal atoms can be investigated.

Previous workers 2-6 have used the method to study the quenching of excited thallium and sodium atoms. The rates of reaction of normal (i.e. in the ground state) sodium atoms with a wide range of organic halides are well-known.7 It seemed to us of interest to investigate the possibility of studying, by the above method, the rates of reaction of excited metal atoms with some of these halides and also, with sodium, to see whether the range of organic molecules could be extended to include types (e.g. alcohols and ethers) which react too slowly with normal sodium atoms to be studied by the conventional sodium " flame " tech- nique. As will be seen from our results, it has not proved possible to study this type of chemical quenching over a wide range of organic molecules. However, the results reported in parts 1 and 2 are a useful addition to the general field of quenching and we have made a number of significant modifications in the technique.

EXPERIMENTAL MATERIALS

B.D.H. thallous iodide was used. Gaseous oxygen, argon and dimethyl ether were used directly from cylinders without further purification. Absolute methanol and ethanol and anaesthetic diethyl ether were used without further purification. The tetra- hydrofuran, di-isopropyl ether and cyclohexane were samples which had been dried and fractionated in an efficient column, the di-isopropyl ether having been previously treated with ferrous sulphate to remove peroxides. The n-hexane was spectroscopic grade which had been passed through a column of silica gel to remove the last traces of benzene. All the quenching substances which were liquid at room temperature were thoroughly de- gassed in the apparatus by several distillations from room temperature to - 196°C.

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D . J . DOWLING AND E. WARHURST 533

APPARATUS AND TECHNIQUE

Fig. 1 is a diagram of the apparatus excluding the electrical circuits which are de- scribed below. The cell C was a 2-cm quartz cube carrying a long side tube, 12 mm in diameter, which communicated with a conventional vacuum line and a storage system containing the quenching substance. This tube was constricted near the cell into a shoulder at J which was ground to fit the nose of a moveable plunger P. The short tube S, 8 cm long and 2-5 cm diam, contained the solid thallous iodide. The cell and salt tem- peratures were measured by the thermocouples TI, T2. During quenching experiments the cell temperatures was kept 30 f 10°C above the salt temperature by means of a small face heater F. The oven consisted of an inconel tube I, 12 cm diam. and 45 cm long. This was wound with nichrome heating tape and was surrounded by the cylindrical asbestos tube A, the intervening space being filled with kieselguhr (asbestos wool in later experiments) to produce efficient lagging. The oven was thermo-regulated by a mercury contact thermometer TR operating a valve relay. L is a quartz lens 5 cm in diameter and approximately 5 cm focal length. To protect the lens and minimize draughts the quartz plate Q was fitted in the position shown in fig. 1. To reduce scattered light from the edges of the cubic cell it was fitted with a metal mask. The atomic fluorescence could be observed through the orifice 0 in the side of the oven ; the light passing through first a Pyrex plate (to exclude draughts) and then a Pyrex lens (for focussing) before emerging from the oven.

FIG. 1.

The source of u.-v. radiation was an a.c. spark between zinc electrodes. The two electrodes were discs 6 cm diam. and 0 6 cm thick with sharp bevelled edges. These were rotated at high speed at right-angles to each other to prevent arcing. This source was moveable in three dimensions for focussing. The power supply, which was similar to that used by previous workers but much less powerful, was obtained from a high-voltage transformer, 8 kV, 2 kW, fed from the as . mains. The current in the primary circuit was controlled by a variable choke. The spark gap and a condenser of 0.05 pF capacity were connected in parallel across the secondary windings of the transformer one side of which was earthed.

Previous workers, with the exception of Hanson,6 used visual methods for measuring the intensity of the atomic fluorescence. We have developed successful photo-electric methods in the two techniques described in parts 1 and 2. Attempts to monitor directly the u.-v. light from the spark using a photomultiplier tube proved unsuccessful. Instead, a beam of the u.-v. light from the spark was selected by an iris diaphragm and a small quartz lens and allowed to fall on a glass plate coated with vaseline impregnated with naphthalene. The visible fluorescent light was arranged to fall on the monitor photo- multiplier which was an R.C.A. 931A. tube. The green atomic fluorescence emerging through the orifice 0 was focussed by the Pyrex lens on the metering photomultiplier tube (also R.C.A. 931A.) which was bolted rigidly to the side of the oven. A Chance O.G.R.l filter, with a peak transmission near to the thallium 5350A line, situated immediately in front of the photomultiplier tube reduced considerably the effect of undesirable scattered light from the oven.

A null method was devised for measuring the intensity of the atomic fluorescence. A common voltage supply to both photomultiplier tubes was obtained from a bank of ten 90-V dry batteries. The anode of the metering photomultiplier tube was connected through afixed anode load of 50 klR to the h.t. supply of + 90 V on the dry battery system.

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534 QUENCHING OF EXCITED ATOMS

The anode of the monitoring photomultiplier tube was connected to the same h.t. point through a variable anode load of maximum resistance 50 kQ. The voltages developed across these two anode loads were applied in opposition across the primary of a trans- former of the usual intervalve type. One end of the secondary of this transformer was earthed and the other end was connected to the Y-plates of a Cossor oscilloscope, the time base being set at continuous traverse. The responses of the two photomultiplier tubes could be balanced by means of the variable anode load and, ideally, the oscilloscope should then show no response. In practice this was never achieved, but it was found that an oscilloscope response of minimum ripple, approximately symmetrical about the base line and which was clearly discernible, could always be easily found. This balance was sensitive to adjustment of the variable anode load and was found to be very reproducible. The spark voltage used was about 5 kV.

During a quenching experiment, the plunger was kept in the closed position as much of the time as possible to reduce the loss of salt by distillation down the long tube to the cold end. The pressures of quenching gas admitted were measured by a manometer. With volatile liquids the appropriate pressures were produced by refrigeration of the liquid in a side-tube attached to the vacuum line. In order to detect systematic errors in the intensity measurements, for example changes produced by a drift in the salt vapour pressure or by a progressive fogging of the cell windows during an experiment, the pressures of quenching gas, from the highest to the lowest values, were chosen in a haphazard manner. The results did not indicate the existence of any errors of this kind. The method of obtaining a value of Q = Io/Z, where 10 is the unquenched intensity of the atomic fluorescence and Z the intensity in the presence of a pressure p of the quenching gas, was as follows. The oven was heated and allowed to come to thermal equilibrium with the cell and salt temperatures 440" and 410"C, respectively. The system was then rapidly pumped down to a pressure of < 10-4 mm. The spark source was focussed and the iris diaphragm in the beam of u.-v. light to the monitoring photomultiplier tube was adjusted until the responses of the two photomultiplier tubes balanced with the variable anode load at its full value of 50 k8 , this value representing 10 + B, where B is the back- ground intensity. After the admission of a known pressure of quenching gas a balance was re-established with a lower value, say x kQ resistance, of the anode load. This represents I + B. The background intensity was determined in two ways. At intervals during an experiment a sheet of glass was interposed between the spark source and the lens L. This completely cut off the u.-v. content of the spark light. The photomultiplier tubes responses were then balanced, the variable anode load being, say b kQ. Then 10/1= (50 - b)/(x - b). The second method assumes that the quenching intensities follow the Stern-Volmer relationship (see later). It may easily be shown that, in terms of our measured quantities, x = (50 - x)/C' + by where C is a constant. Hence a plot of x against (50 - x)/p should be linear with an intercept of b. The two methods yielded values for the background intensity in satisfactory agreement with each other ; the mean value was used in calculating our results. The validity of this method of determining Q depends upon a linear response with light intensity for both photomultiplier tubes. From the tube specifications it would appear that this linearity should hold accurately for the relatively low light intensities which we have used and our results showed no features which would indicate appreciable non-linearity.

RESULTS

The conventional way of presenting quenching results is in terms of the effective collision cross-section. This is denoted by q by Terenin and Prileshajewa 3, 5 and by 4 by other workers, the relationship between these two being q = vu2. We have cal- culated 02 values by the method described by Terenin and Prileshajewa.3 The velocity of an excited thallium atom, produced by the photolysis of a thallous iodide molecule, relative to the parent molecule is

where MI and M2 are the masses of the thallium and iodine atom, respectively and W = hu - (D(T1- I) + ETL) is the excess energy of the u.-v. photon which appears as kinetic energy. v is the frequency of the u.-v. radiation, D(T1- I) the dissociation energy

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D. J . DOWLING AND E. WARHURST 535

of the T1- I bond and Em is the excitation of the thallium atom (62P+ -+ 72Sg). The actual average velocity of the excited thallium atom is

.Z;l = vp + (i9/3vP) when 0; > V,

or 51 = V + (vy2/3G) when vy < V.

where 5 is the average thermal velocity of the thallous iodide molecules. The number of eflecfive (i.e. quenching) collisions 2 of a thallium atom having a specific velocity V l with molecules of a quenching gas which have a Maxwell distribution of velocities and a con- centration NQ molecules cm-3 is

2 = ~/Z+NQUJ$(X),

where

02 is the efective collision cross-section and MQ is the mass of the quenchiag molecule. The well-known Stern-Volmer quenching expressi6n can be cast into the form

where A is the Einstein coefficient of the excited state. A plot of Q against the pressure of quenching gas should, therefore, be linear with an

intercept of unity on the Q-axis. From the slope of this line kr or 2 (and hence 02) can be calculated. The following data were used in the calculations. A is the sum of the Einstein coefficients for the two transitions 72Sh 3 62Ph and 72Sh -+ 6*P3/2 = 7.2 x 107 sec-13 ETI = 76 kcal.8 From the data given by Gaydon 9 and Herzberg 10 we have chosen a value of 59 kcal for D(T1 - I). The average wavelength of the group of lines in a zinc spark which is effective in the photodissociation of thallous iodide is stated to be 5 2040 A.

Our calculated values of 02 and kr are given in table 1. These are discussed together with the results obtained in part 2 at the end of the latter paper. A typical graph of Q against the pressure of quenching gas for methanol is shown in fig. 1, part 2, in which are also plotted the results for the same compound obtained by the single-spark technique.

It turns out that for many of the quenching molecules studied the average number Y of collisions required to bring about quenching is appreciably greater than unity. In

TABLE TH THE QUENCHING OF EXCITED THALLIUM ATOMS AT 440°C

Stern-Volmer d(cm2) a:,(cmz) kr ( x 10-13) quenching gas slope (mm-1) x 1016 x 1016 cm3mo1.-1sec-'

dimethyl ether diethyl ether di-isopropyl ether tetrahydro fur an methanol ethanol hexane cyclohexane oxygen * argon

0.0 12 0.019 0.021 0.041 0.016 0.021 0.007 0.0026 0.01 5-0-022 0.0001

3.1 5.8 7.0 2.3 3.6 5.4 2.2 0.8 3.1-4.9 0.023

3.2 6.0 7.6

3.7 5.6 2.4 0.9 3.5-5.2 0.024

-

3-94 10.9 6.2 5.8 6.9 4.6 3.5 2.8 5-3 9.5 6.9 6.2 2.3 14.7 0.85 40

0.03 1,520 5.0-7.5 10-6.7

* The quenching experiments with oxygen showed a curious feature. The observations for each run gave a good linear plot of Q against oxygen pressure but the slopes of these lines varied from run to run, giving the range of 02 values indicated in the table. We cannot give a satisfactory explanation of this. It was never found for any other quenching substance which we investigated and, in many of these cases, several runs were carried out on different days.

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536 QUENCHING OF EXCITED ATOMS

these instances the excited atom, which initially possess a velocity which is greater than the average thermal value, will become thermally moderated before quenching takes place (see, for example, Pringsheim 11) and it seems to us more appropriate to calculate the cross-section using the conventional collision expression for particles which have a Maxwell distribution of velocities. We have denoted these cross sections as c& in the table; in most instances the values are very close to the corresponding u2 values. For the cases for which we consider that thermal moderation takes place we have calculated Y from Y = U&/U+M and for the remaining cases, i.e. the more efficient quenchers, from Y = U$k/U2, where u$k is the gas-kinetic collision cross-section. The precise values of u2gk are extremely uncertain and to conform to the usual practice in calculating the collision yields of reactions involving normal sodium atoms, i.e. in sodium flame work, we have adopted the standard value of 35 A2. We have taken arbitrarily Y > 5 as a rough guide as to cases in which thermal moderation probably operates.

1 Terenin, 2. Physik, 1926, 37, 98. 2 Winans, 2. Physik, 1930, 60, 631. 3 Terenin and Prileshajewa, 2. physik. Chem. B, 1931, 13, 72. 4 Kisilbasch, Kondratjew and Leipunsky, Sowiet Physik., 1932, 2, 201 ; 1935, 8, 644. 5 Prileshajewa, Actaphysicochim., 1935, 2, 647 ; Sowiet Physik., 1932,2, 351. 6 Hanson, J. Chem. Physics, 1955, 23, 1391. 7 Warhurst, Quart. Reo., 1951, 5, 1. 8 Mitchell and Zemansky, Resonance Radiation and Excited Atoms (Cambridge U.P.,

9 Gaydon, Dissociation Energies and Spectra of Diatomic Molecules (London, 1953). 1934), p. 212.

10 Herzberg, Molecular Spectra and Molecular Structure. I. Spectra of Diatomic

11 Pringsheim, Fluorescence and Phosphorescence (Interscience Publishers, 1949), p. 1 14. Molecules (New York, 1950).

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PART 2.-THE QUENCHING OF EXCITED THALLIUM AND SODIUM ATOMS PRODUCED BY PHOTODISSOCIATION

USING A SINGLE SPAFtK TECHNIQUE

BY D. J. DOWLING, G. R. H. JONES AND E. WARHURST Dept. of Chemistry, The University, Manchester

Received 10th November, 1958

The quenching of excited thallium and sodium atoms produced by the photodis- sociation of the metal iodide vapours has been investigated. The method involves ob- servations with single, triggered sparks of high intensity. The atomic fluorescence in- tensities were measured by a photoelectric method. The cross-sections for the quenching of excited thallium atoms by diethyl ether, methanol and acetonitrile and those for the quenching of excited sodium atoms by carbon dioxide, carbon tetrafluoride, methanol and acetonitrile have been measured. These cross-sections, and those of the preceding paper, have been discussed and mechanisms for the quenching processes in various cases have been put forward.

Attempts to investigate the quenching of excited sodium atoms produced by the photodissociation of sodium iodide using the apparatus described in part 1 proved unsuccessful. The intensity of the atomic fluorescence was too weak, due to an insufficiently powerful energy supply to the a.c. spark, This difficulty was overcome in the method described below which is based on observations with single, triggered sparks of high intensity.

EXPERIMENTAL MATERIALS

B.D.H. thallous iodide and B.P. sodium iodide were used. Anaesthetic diethyl ether and absolute methanol were used without further purification. B.D.H. acetonitrile was dried and fractionated in an efficient column. Carbon tetrafluoride, fluoroform and methylene difluoride were given to us by Imperial Smelting Corporation Ltd. They were each subjected to bulb-to-bulb distillation at low temperatures in the apparatus before use. The sample of C02 was obtained from dry ice, which was sublimed several times from - 120" to - 196"C, a middle fraction being selected.

APPARATUS AND TECHNIQUE The oven and the cell were the ones described in part 1. For work with sodium iodide

the following modifications were made. The operating temperatures were raised to 620°C for the cell and 580°C for the salt tube S (fig. 1, part 1). The optics were improved by using a quartz lens 10 cm in diameter and 10 cm focal length at L, and the amount of scattered light was further reduced by '' tubing " the atomic fluorescence from the cell face to the orifice 0 along a metal cylinder. After leaving 0, the light passed through a Chance filter OY1, to reduce unwanted scattered light, before falling on the photo- multiplier tube.

New methods to supply power to the spark and to measure the intensity of the atomic fluorescence were developed. The 8 kV, 2 k W transformer was fed from a variac trans- former on the ax. mains. The secondary windings of the high voltage were connected in series with a mercury arc rectifier (Ferranti H.G. 43), a controlling resistance and with two spark gaps. One side of this circuit was earthed, the rectifier and resistance being located on the high tension side. A 4pF condenser, an electrostatic voltmeter and a 20 Mi2 resistance were each connected in parallel across the spark gaps. In this way the output of the transformer was rectified and charged up the condenser to 8 kV. The main spark gap, which was used as the source of u.-v. light, consisted of rods of the appropriate metal, 0.4 cm diam. with hemispherical ends serving as the electrode surfaces.

20 537

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538 QUENCHING OF EXCITED ATOMS

The subsidiary spark gap consisted of triple points of tungsten wire. The main spark was triggered in the following way.1 A point between the two spark gaps was connected to a tapping on the 20 MSZ resistance which divided the total voltage of 8 kV into about 6 kV across the main gap and 2 kV across the subsidiary gap. A thyratron, biased to cut off, was connected across the subsidiary gap. By pressing a button switch and earthing the grid of the thyratron the subsidiary gap could be shorted out and the main spark “ fired ” at a controlled voltage. The energy of each spark was estimated to be about 120 J which was about 160 times that of each spark obtained by the a.c. technique described in part 1.

With these single, high-intensity sparks the null method described in part 1 proved unsuccessful for intensity measurements. Instead, a much simpler technique was devised. The response from the metering photomultiplier tube was applied directly to the Y-plates of a single beam Solartron oscilloscope (CD 514). This oscilloscope had a the-persistent screen and was used with the time base triggered by the pulse to the Y-plates. The atomic fluorescence produced by each spark gave a clear trace on the screen with a well-defined peak the height of which could be measured with an accuracy of k 3 % using a graduated scale. Careful measurements of large numbers of peak heights for sparks “fired” at a constant voltage showed that these were surprisingly reproducible, e.g. the heights for a dozen consecutive sparks showed deviations of less than 5 % from the mean. This obviated the need to monitor the intensity of the u.-v. light from the spark.

The technique of a quenching experiment was the same as that described in part 1 with the exception of the method of determining Q. Differences in light intensity were not measured by differences in peak heights. The Y-plate sensitivity control of the oscillo- scope was always adjusted to give nearly constant peak heights of about 6cm, small differences being a matter of minor corrections. The Y-plate sensitivity control was calibrated using a series of known voltages. In order to minimize the effect of small variations in the spark intensity the observations were always carried out in the following order. First, a series of readings with no quenching gas present, corresponding to 10 + B, was taken, then a similar series with a sheet of glass in front of the lens L, corresponding to B, followed by a series with a known pressure of quenching gas in the cell, corresponding to I + B, and finally a second series of readings with no quenching gas present. For each of these sets of readings the Y-plate sensitivity control had been adjusted to give peak heights about 6 cm and each set comprised observations on ten consecutive sparks. Thus for each pressure of quenching gas the observations for I were always sandwiched be- tween two sets for I0 and one for the background. The mean peak heights of the sets of ten readings were calculated and these, together with the corresponding values of the Y-plate sensitivity control, enable the readings to be converted into absolute volts from which Q can readily be determined. This method of determination of Q, in addition to the assumption concerning the linearity of response of the photomultiplier tube (which we have dealt with in part 1) also involved the assumption that the peak height on the oscilloscope is an accurate measure of the maximum intensity of the atomic fluorescence produced by the spark. We consider that the good agreement (see below) between the values of the quenching cross-section obtained by the methods of parts 1 and 2 for methanol and diethyl ether shows that the above assumption is justified.

RESULTS The values of k,, 02, O& and Y are given in tables l(a) and (b). They have been

calculated by the method described in part 1. The following data were used. For the photodissociation of thallous iodide the mean frequencies of the active u.-v. lines are 2040, 2020 and 1960 A for zinc, iron and aluminium spark sources, respectively 2 and for sodium iodide the frequencies are 2400, 2082 and 1990A for iron, zinc and aluminium spark sources, respectively2 For the evaluation of W, the excess energy of the u.-v. photon, we have 4 used D(Na-I) = 71 kcal mole-1 and = 48.4 kcal mole-1 for the excitation energy ( 3 2 s ~ 3 32P*, t) . A value of 6.25 x 107 see-1 was used for the Einstein coefficient A for the excited sodium atom.3 A typical plot of Q against the pressure of quenching gas is shown in fig. 1 for the experiments with methanol. This graph also shows the very good agreement obtained by the methods of parts 1 and 2 for this case.

It is clear that, unless corrections are made, this method of studying quenching utilizing the photodissociation of molecules will give spuriously high cross-sections if the quenching gas absorbs the u.-v. radiation to an appreciable extent. We have measured the absorp- tion spectra from 2500 A down to 2000 8, of carbon tetrafluoride, methanol, acetonitrile,

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D. J. DOWLING, G . R. H . J O N E S A N D E. W A R H U R S T 539 and diethyl ether at appropriate pressures at room temperature using a Perkin-Elmer model 4000 u.-v. spectrophotometer. Although proper corrections require absorption spectra information at the cell temperatures which we have used, the spectra at room temperature have been used to make small corrections of a few per cent only to our results for the quenching of thallium atoms by methanol using zinc and iron spark sources and for the quenching of sodium atoms by methanol using a zinc source. Larger, and much less certain, corrections have been applied to the results on the quenching of

4 -

Q

I I F I I I

-

zinc

-

I I I 1 t I

pressure of methanol vapour, mm. FIG. 1.-The quenching of excited thallium atoms by methanol (zinc spark source).

0 Dowling and Warhurst. 0 Dowling, Jones and Warhurst.

TABLE lG.-THE QUENCHING OF EXCITED THALLIUM ATOMS AT 440°C

quenching spark gas source

diethyl ether Zn Fe A1

methanol Zn Fe A1

acetonitrile Zn Al

Stern- Volmer

slope (mm-1)

0.022 0-024 0-032 0-016 0.01 8 0.019 0.066 0-064

02 (cmz) x 1016

6.8 7.1 8.8 3.6 4.0 - 4.0

16.2 14-8

.Ibf (cmz) x 0116

7.2 7.8

10.4 3.7 4 2 - 4.4

k, ( x 10-13) cm3 mole-1 sec-1

7.3 7.9

10.6 5.3 5.9 - 6.3

21.8 21.1

TABLE lb.-THE QUENCHING OF EXCITED SODIUM ATOMS AT 620°C

quenching source gas spark

carbon dioxide Fe Zn

carbon tetra- fluoride Fe

Zn A1

methanol Fe Zn

acetonitrile Fe Zn

Stern- Volmer

slope (mm-1)

0-34 0-25

0~0009 0.0012 0.0018 0.038 0.03 1 0-50 0.30

az(cmz) x 1016

74 18

0.23 0.09 0.12 7.5 2.3

107 22

0.16 0-32 0.21 0-42 0-32 0.64 5 -9 13-3 4.8 10.9 - 170 - 110

Y

4.9 4.5 3.4 9.5 8.3 - 8-0 2.2 2.4

Y

< 1 1.9

219 166 110

5.9 7.3

1.6 < 1

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540 QUENCHING OF EXCITED ATOMS

thallium atoms by methanol using an aluminium spark source and our value of a2 is there- fore shown in table 1 as an approximate figure only. There appears to be no evidence which indicates that appreciable corrections are needed for any of the other quenching gases used.

DISCUSSION

The values of 02 obtained by the two methods for the quenching of thallium atoms by methanol (3.7 and 3.7A2) and by diethyl ether (6.1 and 7.2A2) are in good agreement, particularly for the former compound.* This indicates strongly that neither method involves serious inaccuracies. Only a few comparisons with the results of earlier workers can be made. From Hanson’s 5 results for the quench- ing of sodium atoms by carbon dioxide, the values of 02 corresponding to the use of iron and zinc spark sources can be calculated. These are 28 i%2 (Fe) and 10 A2 (Zn). The value obtained by Winans 6 for a cadmium spark source, when re- calculated 5 by the method used in the present work is 19.5A2. Our values of 74 i%2 (Fe) and 18 A2 (Zn) show the same gradation with changes in the wavelength of the u.-v. radiation but are about a factor of two larger. Considering the diffi- culties in the different techniques the agreement is regarded as reasonable. Prileshajewa 7 gives values of q (= m 2 ) for the quenching of thallium atoms by several compounds, including oxygen and argon. She states that the value for oxygen is dependent on the wavelength of the u.-v. radiations but, unfortunately, she does not specify the wavelength corresponding to the quoted value of the cross-section. This may partly account for the discrepancy between her value of 02 of 11 A2 and ours of 4.0 rt: 0.9 I$ (see table 1, part l), although we believe that the main reason is probably connected with the unexplained day to day variation in 02 which we found (see footnote to table 1, part 1). Prileshajewa’s value of 0.09 A2 for argon is also larger than our value of 0.024 A2. This discrepancy may be due to the fact that small errors in the intensity measurements clearly have a large effect on the very small slope of the Stern-Volmer plot for a very weakly quenching gas.

One of the difficulties which besets the interpretation of the experimental values for quenching cross-sections is the unambiguous identification of the responsible process in individual cases (see, for example, ref. (3), pp. 226-228). As already indicated, our main interest in this field centred round the phenomenon of chemical quenching by the sodium flame type of reaction and it is relevant to examine this process more closely. It has been shown,s on the basis of the theory developed by Eyring, Polanyi and Evans, that the activation energy of the re- action Na + X-R + Na+X- + R may be calculated very approximately by locating the transition state of the reaction at the intersection of the potential energy curve representing the X-R bond extension with that representing the repulsion between the X- ion and the R radical. These curves correspond to a cross section of the complete potential energy surface at the normal internuclear separation for the gaseous ion pair, Na+X-. For the same reaction in which the sodium atom is electronically excited by an amount XNa, this excitation energy would result in a lowering bodily (by an amount XN,) of the repulsion curve relative to the extension curve, with a consequent increase in the exothermicity of the reaction and a reduction in the activation energy by an amount less than XNa. If the normal sodium flame reaction has a low activation energy (as, indeed, is found in many cases) then it is possible that, for the reaction involving an excited atom, this lowering is so large that the two curves do not intersect for the above ion pair Na . . . X separation. However, the repulsion curve can be raised by taking cross-sections at Zarger Na . . . X separations and it is clear that one

* In the following discussion, unless otherwise stated, all our values of 02 quoted are those obtained from the quenching experiments using the lowest frequency of the exciting radiation, i.e. they correspond to the least kinetic energy of the excited atom.

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D . J . DOWLING, G . R . H . JONES A N D E. W A R H U R S T 541

separation can always be found for which the repulsion curve cuts the extension curve at the minimum of the latter. This reaction path would clearly have zero activation energy and would lead to a vibrationally excited Na+X- molecule. This corresponds to very efficient chemical quenching characterized by an en- hanced collision cross-section.

In discussing the results which we have obtained, our conclusions concerning the likelihood of chemical quenching by the above mechanism being the responsible process are based mainly on the following considerations, The heats of reaction -AH and the activation energies E for the reactions between normal sodium atoms and the methyl halides are well known.9 These results satisfy the empirical relationship - AH + E = 22 f 3 kcal. Taking this as a rough generalization, we conclude that chemical quenching with a very low activation energy is a likely mechanism only when the reaction is more exothermic than about 20 kcal and for cases in which - AH is less than this the above mechanism can be ruled o:it fairly confidently. This rough guide, however, may not apply when the exo- thermicity of the reaction is so great that it exceeds the dissociation energy of the

TABLE 2.-HEATS OF REACTION * (kcal

reaction Dn (MX)g D (x--R)g - A H

Na* + N-C-CH3 -+ NaCN + CH3 83(4 103 28 Na* + HO-CH3 -+ NaOH + CH3 74w - 90 32

Na* + HO-H + NaOH + H 74m 116 6 Na* + O=C=O -+ NaO + CO 78(4 127 - 1

Na* + F-CF3 -+ NaF + CF3 107 121(4 34 T1* + H-CH3 -+ TlH + CH3 46 101 21

* - AH = D,(MX), - D(X-R), + X,, where Dn(MX)g is the heat of dissociation of MX into neutral atoms, D(X-R), is the dissociation energy of the X-R bond (the subscript g indicating the gas phase) and Xm is the excitation energy of the metal atom.

In making these estimates of AH, all the dissociation energy values were taken from Cottrell4 except where indicated. Values of the electron affinities E of CN, 0, OH and F and the lattice energies UO of alkali metal cyanides and hydroxides were taken from Pritchard.11

(a). D,(NaCN), = Di(NaCN), + ECN - IN*, where Di(NaCN), is the heat of dis- sociation into ions. Dj was estimated from the relationship - Uo/Di = 1.44. This relationship holds very well for the alkali metal halides and it can also be supported by theoretical arguments.

(b) D,(NaOH), has been estimated by the two following methods; the results agree with each other to within 3 kcal.

(i) Assuming Dj(NaOH), = Di(NaF), on the basis of the close similarity of ionic

(ii) By the method (a) above, i.e. using - Uo/Di = 1.44. (c) Assuming Di(Na0) = Dj(NaF) as in method (b) (i). ( d ) Mean of the values given by Reed and Seddon 12 and Farmer, Lossing, Henderson

radii of OH- and F-.

and Marsden.13

NaX molecule, which would be dissociated immediately on formation (unless it was produced in an electronically excited state). This does not apply to any of the cases which we consider.* Approximate values of AH for the reaction M* + X-R 3 MX + R, where M* is an excited sodium or thallium atom, can be estimated for a number of the quenching molecules, X-R, which we have studied. These are given in table 2. Water has been included for comparison with the alcohols.

* Kondratjew and Siskin 10 have discussed the cross-sections for the quenching of sodium by 0 2 , N2, H2, NO, CO and I2 in terms of the heats of reactions of the sodium flame type. Many of these reactions,' however, are insufficiently exothermic to be likely processes in our opinion.

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542 QUENCHING OF EXCITED ATOMS

For sodium and methanol the evidence points fairly strongly to quenching by the sodium flame reaction. The value of - AH is sufficiently large and, in addition, it is of significance that the cross-sections (- 0.2 A2) found 14 for the quenching of sodium by saturated organic molecules of about the same degree of complexity as methanol (e.g. methane and ethane) are about 30 times smaller than our value for methanol. These hydrocarbons almost certainly quench by a physical mechanism, for example, a transfer of electronic to vibrational energy and not by chemical reaction. Our value of 5.0 1$2 for the quenching cross-section of methanol corresponds to an activation energy of about 3 kcal. Water is a significant contrast to methanol. Hanson 5 reports that water is a poor quencher of sodium atoms. We consider that this is due to the fact that chemical quenching does not occur in this instance ; the reaction is not sufficiently exothermic because of the much larger value of D(H-OH) compared with D(CH3-OH). We think that it is very probable that this type of chemical quenching is also operating in the quenching of thallium atoms by alcohols and ethers. All the relevant chemical reactions would be more than 20kcal exothermic if the values of D(T1-OH) and D(T1-OR) were greater than about 35 kcal, which seems extremely likely. The quenching cross-sections are also about the same as that found for methanol with sodium.

For the quenching of sodium atoms by carbon tetrafluoride the chemical reaction is sufficiently exothermic to suggest that this could be the responsible process. However, the value of the quenching cross-section (0.16 1$2) is very similar to those found for methane and ethane 14 and we are of the opinion that physical quenching is probably taking place in this instance. It is of interest to note that, whatever the quenching process may actually be, our result shows that the decrease in activation energy of the sodium flame reaction on changing from normal to excited sodium atoms must be < 5 kcal, a surprisingly small decrease. on the basis of the well-known relationship for the sodium flame reaction in the case of chlorides 15 AE = uAH, a - 0.27, a much larger change in activation energy would be expected for a change of 48 kcal in the heat of reaction.

It appears to us that there are two equally plausible mechanisms which could account for the very efficient quenching (02 = 107 A2) of sodium atoms by aceto- nitrile. The value of - AH = 28 kcal suggests chemical quenching. The marked decrease in the value of 02 with increasing frequency of the photodissociating radiation (i.e. with increasing kinetic energy of the sodium atom) may be further support for this view. Prileshajewa 7 has suggested that this behaviour is a criterion for chemical quenching with no activation energy. However, acetonitrile is an unsaturated molecule, probably possessing an appreciable electron affinity, and the formation of an ionic “complex ” of the type CH3-C = N-Na+ by an electron transfer seems a very likely possibility. It also does not appear un- reasonable to envisage that the ultimate fate of this complex could be the formation of a normal sodium atom and a vibrationally excited acetonitrile molecule due to the system crossing to a homopolar potential energy surface. This type of mechanism has been suggested by Laidler and Shuler 16 to account for the efficient quenching of sodium atoms by olefines. We also consider that this ionic mechan- ism is probably responsible for the very efficient quenching (02 = 74 A2) of sodium atoms by carbon dioxide ; in this case the ionic complex would be O= C-0-Na+. The value of AH shown in table 2 excludes the sodium flame reaction as a reason- able possibility. On the basis of the value of AH for the reaction

we suggest that chemical reaction is the responsible process in the quenching of thallium atoms by saturated hydrocarbons, as is known to be the case in the quenching of cadmium atoms.17

We attempted to widen the field of investigation with excited sodium atoms by using as quenching gases a number of saturated halides for which the rates of

..

. 0 .

T1’ + HCH3 + TlH + CH3

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D . J . DOWLING, G . R . H . JONES AND E. WARHURST 543

reaction with normal sodium atoms were known. The halides used were ethyl chloride, sulphur hexafluoride, methylene difluoride, fluoroform and cyanogen. In all cases chemical reaction occurred in the cell or salt chamber at 6Oo0C, probably involving exchange of halogen atoms between the sodium iodide and the halide and considerable quantities of free iodine were produced. Since iodine is one of the most efficient quenchers known,lS this would lead to spuriously high cross- sections for the halide molecules.

We should like to thank Imperial Smelting Corporation Ltd., for the gift of methylene difluoride, fluoroform and carbon tetrafluoride.

1 Bardocz and Klatsenanyi, Rev. Sci. Instr., 1955, 26, 947. 2 Prileshajewa, Sowiet Physik, 1932, 2, 351. 3 Mitchell and Zemansky, Resonance Radiation and Excited Atoms (Cambridge U.P.,

4 Cottrell, The Strengths of Chemical Bonds (Buttenvorths, 1954). 5 Hanson, J. Chem. Physics, 1955,23, 1391. 6 Winans, 2. Physik, 1930, 60, 631. 7 Prileshajewa, Acta physicochim., 1935, 2, 647. 8 Evans, and Polanyi, Trans. Furuday Soc., 1938, 34, 11. 9 Warhurst, Quart. Rev., 1951,5, no. 1 . 10 Kondratjew and Siskin, Sowiet Physik., 1935, 8, 644. 11 Pritchard, Chem. Rev., 1953, 52, 529. 12 Reed and Seddon, Truns. Faraday SOC., 1958,54, 305. 13 Farmer, Lossing, Henderson and Marsden, J. Chem. Physics, 1956, 24, 348. 14 Norrish and MacF. Smith, Proc. Roy. Soc. A , 1940, 176, 295. 15 Butler and Polanyi, Trans. Faruduy Soc., 1943,39, 19. 16 Laidler and Shuler, Chem. Rev., 1951, 48, 153. 17 Steacie and LeRoy, J. Chem. Physics, 1944, 12, 34. 18 Terenin and Prileshajewa, 2. physik. Chem. B, 1931, 13, 72.

1934), p. 208.

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