22 needed in this type of quantum yield determination ,

Experimental

With the automatic digitalisation procedure already described it is a quick, easy process to obtain the area under the corrected emission

curves. Before measuring any unknown quantum yields the method was tested by comparing the quantum yields of well-known standards. Comparison of the fluorescence quantum yields of quinine sulphate and anthracene;-

A weighed amount of quinine sulphate was dissolved in 0.5 M H^SO^ and the solution's optical density at 366 nm measured. Anthracene was dissolved in redistilled ethanol and its optical density, also at 366 nm, measured, Both solutions were then diluted to give equal optical densities

at 366 nm of less than 0,02 (to avoid the inner filter effect). The anthracene solution was degassed. The emissions of both these solutions

excited by 366 nm radiation were then recorded and the corrected spectra and area under the curves obtained from the computer printout. Blank

solutions were treated in an identical fashion and the area of scattered light etc. subtracted from the emission area,

22

Taking a value of 0.55 for the quantum yield of quinine sulphate in

22

0.5 M HgSO^ a value of 0,28 (lit. 0.29 ) was obtained for anthracene in ethanol using equation 2,4. This was repeated several times using in some cases solutions of anthracene and quinine sulphate of different

optical densities and making the appropriate correction. The quantum yields obtained were found to be reproducible within t 5%.

4. Triplet-State Absorption Spectroscopy (a) Introduction

Triplet-triplet absorption occurs when the lowest triplet is excited to higher triplet levels by radiation. This is observable in many cases because the lifetime of the lowest triplet is sufficiently long for an appreciable concentration of molecules to exist in the triplet state.

The observation of triplet-triplet absorptions is commonly accomplished by flashing a sample in fluid solution, whence either the absorption or the spectral decay processes can be monitored. The term flash photolysis is used to describe this method of i rradiating a sample by high intensity light flashes of short duration and studying the effects at various wavelengths and times.

The experimental method of flash photolysis has been described in detail and only a brief description is given here. Basically two methods

121-125

are used :

(a) The flash photographic technique; This is used for the detection of short-lived species using a spectrograph and photographic plate. Information covering a large range of wavelengths at one specific time is obtained by this technique.

(b) The flash photo-electric technique; This is used for the measurement of light absorption changes in the sample by means of a

photomultiplier and gives information over a limited range of wavelengths during a specific time interval.

In method (a) a second discharge (spectroscopic flash) is used to produce a photograph via the spectrograph with the absorption of the sample shown at the time when the second flash is initiated. Hence by varying the delay between the photolytic flash and the spectroscopic flash one can study the absorption at a series of wavelengths at specific times after excitation of the sample.

In method (b) a continuous source is used to monitor the absorbance at an appropriate wavelength. Ideally the only light reaching the

photomultiplier should be from the monitoring source, but due to the physical arrangement some of the light from the photo-flash is scattered back into the photomultiplier, thus producing a pulse of

equal duration to the flash and this will mask any transient produced which has a shorter lifetime; therefore the shorter the flash duration the

53

was particularly marked in some of the cases studied here and all the flash photolysis results reported were in fact obtained using the

spectrographic arrangement (a) for which the apparatus used is described below.

(b) General Description of Instrument

All flash photolysis experiments were performed on a Northern

Precision FP-I-pH apparatus. Although the apparatus permitted the sample cell to be held in either a vertical or a horizontal position, the former was always used and the instrumental lay-out for this is shown in FIG. 2,8.

The spectrograph (1) is positioned on the main (photolytic) capacitor case (2) which also supports the sample holder assembly (3). The

spectroscopic flash lamp is housed in (4) and the spectroscopic capacitor case must be placed nearby (5), The power supplies (6), control units (7), (8) and delay unit (9) are placed on a table (10) beside the main capacitor banks. With this arrangement several mirrors (contained in a

flexible light guide) are used to drive the beam onto the spectrograph slit. This^is useful only in the visible regions of the spectrum and for shorter wavelengths the beam must be focussed on the slit by means of a quartz lens.

The main capacitor consists of two banks of five microfarad capacitors which can be operated singly or together. Normally both capacitors were charged to 10 kV before triggering, thus on firing

giving a flash energy of 500 J. The type of switch used for the photoflash is the simplest type, of just two contacts and a connecting bar operated by a solenoid, which operates when the trigger button on the control unit is pressed. This method of triggering is used because of its simplicity, direct action and lack of electrical interference but was found in practice to cause several problems. When firing the photolytic flash large fields are produced and therefore the spectrographic unit must have a high

interference immunity. This is achieved by using a device known as an ignitron which gives reliable triggering of the spectrographic flash.

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