STUDIES I X T H E EXPERIRIESTAL TECHNIQUE OF PHOTOCHEMISTRY
VI. The Energy Distribution of the Uviol Lamp BY E. BEESLEY AND H. N. RIDYARD
Introduction The Uviol lamp, formerly used as the light source in many researches in photochemistry, was the subject of energy distribution determinations by Allmandl, who also described previous attempts in this direction. Owing to the low intensity of the radiation from this lamp, and troubles due to the unsteadiness of the Paschen galvanometer used, the deflections he was able to obtain were very small, and the possibility of error correspondingly large. Although this lamp is not likely t o be much used in future, it seemed to be of interest to redetermine its energy distribution, as a much more sensitive arrangement of apparatus was available in this laboratory. I t was hoped that the results obtained might usefully be applied t o some of the published work in which this lamp was used. An examination of these papers convinced us, however, that too many assumptions would be involved in respect of transmissions of filters and absorptions of solutions under investigation to make this worth while. Experimental The arrangement of apparatus used was essentially similar to that described by Allmand (loc. cit.) and by Franklin, Maddison and Reeve2in Part I1 of this series. It consisted of a Hilger Monochromatic Illuminator for the Ultra-Violet, with a water-cooled shutter before the collimator slit, and provided a t the telescope slit with a Hilger linear thermopile, which was connected t o a Paschen galvanometer. The setting up of the apparatus had, however, been greatly improved, particularly with regard to the Paschen galvanometer, which now rested upon a concrete block, weighing nearly a ton, and standing upon a mound of loose earth in a pit, with no contact with the floor of the building. The shielding was the same as that described by Franklin (loc. cit.) but as all work was carried out a t night, when traffic disturbances were at a minimum, the galvanometer was much steadier. Indeed, an unusual combination of circumstances gave perfect steadiness on one or two nights, enabling small readings to be made with great accuracy. A new suspension had been fitted to the galvanometer, giving about twice the sensitivity previously obtainable. The installation of a Tirrill regulated motor-generator also enabled a constant voltage supply ( I I O volts) to be maintained. J. Chem. SOC., 107, 682 (191j). Chem., 29, 714 ( 1 9 2 j )
* J. Phys.
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EXPERIMENTAL TECHSIQUE O F PHOTOCHEMISTRY
The worst difficulties lay with the lamp itself. This was burnt for the main determinations a t 2.42 amps, 29.5 volts, as these were the most stable conditions. It was found that the lamp frequently deposited thin films of mercury on the glass, thus changing the energy distribution, that it frequently went out, and that as it had to be burnt close to the shutter, in order to get as high an intensity as possible, the slightest change in position caused a disproportionately large change in the intensity of any line being examined. Hence the method was adopted of first examining each line a t 1-2 p p intervals', and then comparing the deflections given by the peaks of the various lines with that given by one line (436 pp) which was taken as standard. A s many lines as possible were thus compared in each position, and the comparison made complete by bringing all the values to a common basis of 436 p p = IOO mm. Each line was examined a number of times. Finally an attempt was made to examine variation of distribution with current density. This was not very successful, owing to the limited stability of the lamp, but a few results are given. The number of deflections measured for each line in each comparison varied from six to twenty. Under the best conditions it was possible to repeat' a reading amounting to a few millimetres six times or more with less than .j mm. difference. The maximum deflection obtained in any case was 8 0 mm. One of us (H. K. R.) in collaboration with D. W. G. Style, has recently published in this series* an account of reflection losses in the spectrometer used, and the results given in Tables I and I1 are corrected by the factors in that paper. Taking 436 p p as tained:579 PP 16 16 546 405 365 313 303
97 64 30 I8
104 72 30
IOO
Results mm., the following peak comparisons were ob38 126 69 31
47 93 68 33
I02
37
36 115
71
71
79
69
35
20
2
Mean values are given in Table I (corrected). The deflections obtained in the various portions of each wave-band were then compared with these peak values, and the results plotted in a graph (Fig. I ) . From the graph the area of the diagram of each line was found, and this divided by dX (the wave length range included by the telescope slit a t this wave length), should give the true energy content of each line (et. Franklin, etc.: op. cit., 716). The results are given in Col. 3 of Table I, and it will be seen that they closely correspond with the peak deflections given above. See Reeve: J. Phys. Chem., 29, 40 (1925). .l Phys. Chem., 32, 861 (1928).
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I n Col. 4 are given the results of Allmand, for comparison. These have also been corrected for reflection losses in the spectrometer, as this was an instrument of the same make, and probably the mirror was similar. I n any case, the corrections are very small. I n Col. 5 are given, for comparison, figures for one of the quartz mercury lamps which have been examined in this laboratory.
FIG. I The Energy Distribution of the Uviol Lamp burning a t 29.5volts, 2.42 amps.
TABLE I Wavelength
579 PP s 46 436 40 36s 3'3 303
s
Area
Quartz mercury lamp
dA
Allmand's figures
36
34
27
115
I 06
99
72
I10
IO0
IO0
IO0
IO0
71
76
57
32
33
42
46 146
20
16
18
Peak comparisons
76 34
2
These results are a t 2 . 4 2 amps. and 2 9 . 5 volts, except Col. 4 (3.2 amps. and 3 4 volts.). The effect of change of amperage is shown in Table 11. (corrected). TABLE I1 Wavelength 5 7 9 PP
Peak 1.5 A. 30V.
Comparisons 3.5 A. 30.5 V.
546
93
37 I08
43 6
IO0
IO0
29
Wavelength 405
365
Peak Comparisons 1.5A. 30 V. 3.5 8. 30.5 V.
68 27
31
It will be noticed that there is practically no change in peak values when the current is raised from 2.42 to 3.5 amps. This agrees with Allmand's results.
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Summary The energy distribution of the Uviol Lamp has been examined, and the results compared with earlier work and with the energy distribution of the Quartz Mercury Lamp. The authors wish to express their thanks to Professor A.
