Mercury absorption spectrum

One of the first applications of this chopped-beam irradiation technitriplet spectra was reported by Labhart From a knowledge of the intensity of the irradiation light, he determined the quantum yield of triplet generation to be 0.55 0.11 for outgassed solutions of 1,2-benzanthrazene in hexane at room temperature. Hunziker 32) has applied this method to the study of the gas-phase absorption spectrum of triplet naphthalene. A gas mixture of 500 torr Na, 0.3 mtorr Hg, and about 10 mtorr naphthalene was irradiated by a modulated low-pressure mercury lamp. The mercury vapor in the cell efficiently absorbed the line spectrum of the lamp and acted as a photosensitizer. The triplet state of naphthalene was formed directly through collisional deactivation of the excited mercury atoms. 

Figure 19 shows the ultraviolet absorption spectrum of a typical diazonaphthoquinone and a common novolac resin. The naphthoquinone sensitizer has a strong absorbance at the 365 nm., 405 nm., and to a lesser extent the 436 nm. mercury emission lines. There are two diazonaphthoquinone isomers that are used in commercial photoresist formulations that are available at this time. The 5-arylsulfonates are by far the most commonly used. A spectrum of a representative of this class of materials is depicted in Figure 20. The 5-arylsulfonate materials are characterized by an absorbance maximum at approximately 400 nm. and a second, slightly stronger maximum at approximately 340 nm.

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

Fig. 4. The absorption spectrum of a crystalline film of salicylidene-2-chloroaniline before (full curve) and after (broken curve) irradiation. (Temperature —131 °C. irradiation 23 min. through Corning filter F 5874, 250-w. high pressure mercury arc). See ref. 21.

The solution of C60H18 has been irradiated inside a quartz reactor with a low pressure mercury lamp having a monochromatic emission at 245 nm. The solution was kept under continuous He blanket to avoid any interference from air. Periodically samples from the irradiated solution were taken to measure the electronic absorption spectrum. 

Fig. 16. The appearance of rotatory artifacts both at the minimum in the emission spectrum of the mercury arc and as increasing polypeptide absorption is encountered below 250 m/i. The rotations are those actually observed, a, in degrees, for a 0.6% poly-L-glutamic acid solution at pH 7 in a 10-cm cell. The absorption spectrum of poly-L-glutamic acid and the emission spectrum of the arc, uncorrected for detector response, are in arbitrary units. (Urnes et al., 1961b.)...

Relationship of absorption spectrum to wavelengths of mercury lines. 

Quite a number of workers have examined specific aspect of the direct photolysis of substituted benzenes and, although the mechanisms are not fully understood, some important conclusions have been reached. Hentz and Burton examined the photolysis of toluene, ethyl benzene and mesitylene in both liquid and vapor states using a medium-pressure mercury lamp. They concluded that the gas-phase products, hydrogen, methane and ethane, were formed with a quantum yield of about 10 , while polymer formation was much more important. At 150 °C hydrogen was the most important gas-phase product except for the case of ethyl benzene in the vapour, where both methane and ethane were more important than hydrogen. Porter and Wright have shown by flash photolysis that benzyl radicals are formed in the photolysis of toluene and ethyl benzene and have observed the absorption spectrum of the benzyl radical. 

Whenever possible, essentially monochromatic light sources such as low- or medium-pressure mercury arcs equipped with bandpass filters or a monochromator (Figure 3.28), narrow-band photodiodes or lasers should be used for quantum yield determinations, because quantum yields can only be defined for monochromatic irradiation. This can be relaxed if the absorption spectrum of the actinometer is close to that of the sample (Section 3.9.6). One then assumes that the quantum yield is independent of the wavelength of irradiation. The stability of the light source over time is essential. Medium-pressure mercury arcs that have a stable output for many hours after a burn-in period of about 30 min are available. Xenon arcs tend to fluctuate abruptly when the arc between the electrodes jumps from one position to another. Intensity fluctuations of the light source in time can be monitored with photodiodes. This should be routinely done with pulsed lasers. 

Isomerization of the polymer from a predominantly cis configuration to a predominantly trans configuration could be accomplished either thermally or photochemically. Thermal isomerization was accomplished by heating the sample in benzene or THF at 60-80°C in a tube sealed with a Teflon Kontes screw-top until the visible absorption spectrum showed no change. Photochemical isomerization was accomplished at 0°C (monitored by thermocouple and external meter in the bath) by exposure of the sample dissolved in THF or benzene to light from a Pyrex-filtered, 350 W, medium-pressure mercury Hanovia lamp (approximately 6-12 h for a sample concentration of 1 mg mL ). Overexposure resulted in a decrease in color indicating decomposition of the material in solution. THF, toluene and benzene were suitable solvents for this experiment. Chlorinated solvents sometimes lead to photobleaching. 

The equipment used was a Barringer airborne mercury spectrometer (BAMS, Figure 1), an atomic absorption spectrophotometer specifically designed and built to isolate the 2536.5-A emission and absorption spectrum characteristic of atomic mercury vapor. The equipment was originally developed for mineral exploration purposes and for analysis of laboratory soil samples subsequent design improvements led to the rapid response time (1 sec) and high resolution (nanogram per cubic meter) required for airborne use (1). 

The imaging performance of poly(vinyl cinnamate) when exposed by a medium-pressure mercury arc lamp is poor. This is due to the mismatch between the absorption spectrum of the cinnamoyl group (with absorbance maximum at 280 nm) and the spectral emission of the mercury arc. The absorption spectrum of poly(vinyl cinnamate) does not overlap with most of the strong emission lines of a mercury arc lamp. This problem can be overcome by spectral sensitization, for example, with the addition of 5% of Michler s ketone, or by the replacement of the cinnamoyl group with a chromophore such as in poly(vinyl cinnamylidene acetate) (IV) that absorbs at longer wavelengths. ... 

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