HANDBOOK OF PHOTOCHEMISTRY, THIRD EDITION

11
Light Sources and Filters
11a SPECTRAL DISTRIBUTION OF PHOTOCHEMICAL SOURCES
This section gives a brief survey of the spectral distribution in a variety of photon sources that are useful for photochemical studies; see also ref. [8201] (Chapters 3 and 16) and [8901] (Chapters 5-7).
11a-1 Conventional Light Sources
Tungsten and tungsten-halogen lamps:tungsten lamps emit in a continu-ous manner from the near UV into the IR; in the tungsten-halogen lamps, in which the halogen (usually bromine or iodine) adds chemically to the evaporated tung-sten at the bulb wall, the gas mixture decomposes with redeposition of the tung-sten at the hot filament in a regenerative cycle. The bulb temperature must be high (~250°C) to maintain this cycle; thus, these lamps have small bulbs of quartz or fused silica. Individually calibrated tungsten-halogen lamps are used as standards for calibrating the spectral response of spectrometers.
Arc lamps: another important class of photon sources is that of arc lamps. Deuterium lamps are UV sources used for spectroscopic purposes, that provide an essentially line-free continuum from 180 to 400 nm. The spectral output of xenon lamps consists of a smooth continuum starting from the UV, with a weak super-imposition of lines in the visible, with strong lines in the near IR. Most xenon lamps operate at high pressure (~20 bar) with short arc configurations and use a DC power supply for stable operation, and can be easily used in a pulsed way, such as in flash-photolysis spectroscopic studies. The spectral distribution of a xenon flash lamp depends upon the electrical discharge conditions, however in all cases the output in the UV is increased with respect to the output from a steady state lamp. Xenon-mercury lamps are short-arc lamps with a pressure of ~ 1 bar xenon, which adds a continuum background to the spectral output, which is mainly that of conventional high-pressure mercury lamps (see below). The qualita-tive emission spectrum of a Xenon arc is given in Fig. 11a-1. Comparative spectra are given in Fig. 11a-2 for a variety of arc lamps. These plots show the spectral irradiance as a function of the wavelength (energy or power spectra). In order to convert these spectra to photon spectra, the ordinates must be multiplied by factors
584 Handbook of Photochemistry proportional to the wavelength. According to ref. [8901], page 156, the number of photons emitted by a lamp per second, at a particular wavelength (λ, nm) is related to the optical power as follows:
number of photons (einstein s-1) = power (watts) ×λ× 8,359×10–9 Thus, the reported spectra are greatly exaggerated in the shorter wavelength range if the reader is concerned with number of photons (see, for example, Fig. 11a-1).
Fig. 11a-1. Typical spectral irradiance of a xenon lamp: a) power spectrum of a 150 W lamp (adapted by permission from LOT-Oriel catalogue); b) photon spectrum of a 350 W lamp (reprinted from ref. [6801], page 165, by permission of Elsevier).
Light Sources and Filters 585
Fig. 11a-2. Spectral irradiance of some arc lamp sources. Reprinted by permission from
LOT-Oriel catalogue.
For their relevance in photochemical studies, it is of interest to concentrate
some additional attention to the various mercury lamps available. Low-pressure
lamps operate with a vapor pressure below 1 bar, and close to room temperature.
Lamps operating at about 10-6 bar emit mainly radiation (mercury resonance lines)
at 184.9 and 253.7 nm (intensity ratio=1:10), although the spectral output varies
with the bulb temperature, mercury pressure, and arc current. The 184.9 nm line is
not observed, unless a bulb of suitable UV quartz is used. Almost all low-pressure
arcs operate using an AC voltage supply with a low arc current. Medium-pressure
mercury lamps operate at vapor pressures grater than 1 bar, but are commonly
called high-pressure lamps, especially in Europe. The medium pressure lamps
considered here operate at a pressure in the range 1-10 bar and are distinguished
from high-pressure lamps by the distance between the electrodes. The spectral
distribution consists of lines, together with a very weak continuum background.
The 253.7 nm line is in most cases absent, owing to the self-absorption by the
high concentration of mercury atoms near the bulb walls. Among the commercial
lamps differences exist in design and operating conditions, such us temperature
and pressure, which lead to small differences in spectral output. Almost all me-
dium-pressure lamps operate using an AC supply. High-pressure mercury lamps
586 Handbook of Photochemistry are of two types, short-arc lamps, with pressure of ~2 to 20 bar, and capillary lamps, which operate with a pressure of ~50 to 200 bar.These lamps operate at very high temperatures and water cooling is usually required to prevent melting of the quartz envelope. The spectral output consists of the normal mercury lines, which are temperature and pressure broadened compared to those observed in me-dium-pressure lamps, on top of a stronger background continuum, with very re-duced output at wavelength shorter than ~280 nm. Almost all high-pressure mer-cury lamps operate using a high current DC supply, which results in a more stable arc compared to AC operation.
Fig. 11a-3. Grotrian energy-level diagram for mercury. The 184.96 and 253.65 nm lines are resonance lines and their intensities decrease quickly with increasing mercury pressure due to reabsorption. The 265.4 nm line is not observed in absorption due to its highly forbidden character. Adapted from ref. [6601], page 52, by permission of Wiley.
Light Sources and Filters 587
Since mercury has an emission line spectrum, it is often used as a conven-
mercury, with the indication of the wavelengths corresponding to the various lines
medium-pressure mercury lamps, while Table 11a-1 gives the corresponding in-
tensity of the most important lines.
For terms used to express the emission intensities of the various light
Table 11a-1 Relative Spectral Energy Distribution of Helios Italquartz Mer-cury Lamps.
Low-pressure 15 W Medium-pressure 125 W λ (nm) Spectral
irradiance
(arbitrary units) λ (nm) Spectral irradiance (arbitrary units)
253.7 100 248.2 5.7
296.7 0.74 253.7 9.1
312.9 2.2 265.3 7.4
365.4 3.0 280.4 3.5
404.7 2.2 289.4 2.7
435.8 8.0 296.7 10
546.1 4.8 302.3 16
312.9 33
334.2 7.4
365.4 100
390.6 1.3
404.7 24
407.8 5.4
435.8 37
491.6 1.3
546.1 33
577.0 20
579.1
29 ient wavelength calibration standard. Fig. 11a-3 gives the energy level diagram for
that can be used. Figures 11a-4 and 11a-5 give the emission spectra for low-and
Finally, the solar power spectrum is shown in Fig. 11a-6.
sources and a glossary of terms used in photochemistry see refs. [0401, 0501].
588 Handbook of Photochemistry
Fig. 11a-4. Emission spectrum of a low-pressure mercury lamp Helios Italquartz 15 W.
Fig. 11a-5. Emission spectrum of a medium-pressure mercury lamp Helios Italquartz 125W.
Light Sources and Filters 589
Fig. 11a-6. Solar spectral irradiance. From ref. [6501], reprinted by permission of McGraw-
Hill.
590 Handbook of Photochemistry
11a-2 Lasers
Lasers are commonly used in photochemical research, especially for time
resolved studies. A list of the most common wavelengths of lasers currently used
is given in Table 11a-2. In addition to these, dye lasers provide tunability over
Table 11a-2 Lasers: Common Wavelengths Used in Photochemistry
Laser λ (nm) Laser λ (nm)
F 2 157 Argon ion 488
ArF (excimer) 193 Argon ion 514.5
KrCl (excimer) 223 Nd:YAG (dupled) 532
Ruby (tripled) 231.4 Krypton ion 568.2
KrF (excimer) 248 He-Ne 632.8
Nd:YAG (quadrupled) 266 Krypton ion 647.1
XeCl (excimer) 308 GaAlAs 670
He-Cd 325 Ruby (fundamental) 694.3
Nitrogen 337.1 GaAlAs 750
Ruby (dupled) 347.2 GaAlAs 780 Krypton ion 350.7 Gallium arsenide 904
XeF (excimer) 351 Nd: glass 1060
Nd:YAG (tripled) 355 Nd:YAG (fundamental) 1064
Nitrogen 428 CO 2 10600
He-Cd 441.6
wide wavelength ranges. A few tuning curves are reported in Figures 11a-7
see ref. [8001]; for a wider list of output wavelengths from commercial lasers see
ref. [9701]. For widely tunable laser diodes see ref. [9801] and [9901]. For femto-
second pulses from the UV to the IR, see ref. [9301].
through 11a-10. A plenty of dyes suitable for different ranges of wavelengths is
listed in ref. [8901], Chapter 7. For an extensive listing of gas laser wavelengths
Light Sources and Filters 591
Fig. 11a-7. Tuning curves for flash-lamp pumped dye lasers. Reprinted by permission of
Exciton. Dyes names are those of Exciton.
592 Handbook of Photochemistry
Fig. 11a-8. Tuning curves for Nd:YAG pumped dye lasers. Reprinted by permission of
Exciton. Dyes names are those of Exciton.
permission of Exciton. Dyes names are those of Exciton.
Exciton. Dyes names are those of Exciton.
11b TRANSMISSION CHARACTERISTICS OF LIGHT FILTERS AND GLASSES
Data and spectra included in this section should help facilitate the selection of appropriate filters and glasses for photophysical and photochemical studies.
Filters are an inexpensive substitute for a monochromator and may be used for excluding a region of wavelengths, cut-off filters, or for isolating a more or less wide range of wavelengths, band-pass filters and interference filters.
A variety of glass filters are commercially available and the reader is re-ferred to the manufacturers’ catalogues for details of their transmission curves. Examples of the most important available types of filters are reported in Fig. 11b-1.
Many solution filters have been described in the literature [4801, 6401, 6601, 6801, 8101, 8201, 8901]. Their use is convenient in irradiation experiments, if monochromatic light or a band of wavelengths of noticeable intensity are re-quired, particularly when photochemical reactors are used. The transmission curves of some useful liquid cut-off filters are shown in Fig. 11b-2 and their com-position is reported in Table 11b-1. Some variations in the precise cut-off wave-length may be achieved by varying the concentration and/or the optical path. The user himself is recommended to measure the transmission of the chosen filter, particularly if very low transmission is required at all the wavelengths shorter than the cut-off point.
A variety of solutions of organic dyes and transition-metal salts are suitable as band-pass filters for wavelengths in the UV and visible region (Fig. 11b-3). To isolate a sufficiently narrow band of wavelengths it is customary to combine a broad band-pass filter, having the required long wavelength cut-off, with a suitable short wavelength cut-off filter. This last combination is particularly convenient for isolating lines of a mercury lamp. An excellent compilation is reported in ref. [6601]. Typical transmittance curves of various types of quartz and glass commer-cially available are reported in Fig. 11b-4.
Fig. 11b-1. Typical transmittance curves of the most important types of filters commercially
available (adapted by permission from LOT-Oriel catalogue). Long-pass (cut-off) (a); short-
pass (cut-off) (b); interference (c); broad-band (d).
Fig. 11b-2. Transmission curves of short-wavelength cut-off filters in solution. For the
composition, see Table 11b-1.
Table 11b-1 Short-Wavelength Cut-Off Filters in Solution
Filter
Compound Conc. Notes
1 methanol pure
2 acetic acid 4 M
3 KI
0.17% w/v %T decreases with irradia-tion
4 CuSO 4.5H 2O 1.5% w/v %T sharply decreases be-yond 500 nm; slight %T increase with irradiation
5 Potassium hydrogen phtalate 0.5% w/v inconsistent, but generally significant decrease of %T with irradiation
6 KNO 3 0.4
M 7 KNO 3 2 M e
8 NaNO 2 1% w/v
9 NaNO 2 75%w/v slight %T increase with irra-diation
10 Fe 2(SO 4)3 3% w/v
11 K 2CrO 4 0.1% w/v
12 K 2Cr 2O 7 0.5%
w/v 13 K 2Cr 2O 7 10% w/v
14 Na 2Cr 2O 7.2H 2O 50% w/v
15 Rhodamine B 0.2% w/v
16 Methyl violet 0.02% w/v also transmits below 460 nm
a b all cases water; c 1 cm pathlength, unless otherwise noted; d Notes on stability from ref. [6601]; for more
details, see this ref.; e 2 cm optical path.
Fig. 11b-3. Transmission curves of band-pass filters in solution. For the composition, see
Table 11b-2.
Table 11b-2 Band-Pass Filters in Solution
Filter Compound Conc., solvent d a
(cm)
Notes b
A Cl2c 1.013 bar 4 also transmits beyond
380 nm
B CoSO4.7H2O d7.5% w/v, water 1 do not mix with
NiSO4; if pre-
irradiated, %T in-
creases to almost stable
value
C NiSO4.6H2O d50% w/v, water 1 do not mix with
CoSO4; if pre-
irradiated, %T in-
creases to almost stable
value
D 2,7-dimethyl-3,6-dia
zacyclohepta-2,6-
diene perchlorate e 0.02% w/v, water, 1 constant transmission
with irradiation
E KCr(SO4)2.12H2O d15% w/v, 0.5 M
H2SO41 also has a window of
low transmission in the
visible
F I2c0.75% w/v, in CCl4, 1 also transmits beyond
650 nm; slight %T
increase with irradia-
tion
G CuSO4.5H2O d10% w/v, water 5
H CuCl2.H2O c5% w/v, 8 M HCl 1
a b c
d From ref. [4801];
e From ref. [9301].
Fig. 11b-4. Typical transmittance curves of various types of quartz and glass commercially available (adapted by permission from LOT-Oriel catalogue). The external transmission
includes the surface reflection losses.
REFERENCES
[4801] Kasha, M. J. Opt. Soc. Am. 1948, 38, 929-935.
[6401] Pellicori, S. F. Appl. Opt.1964, 3, 361-366.
[6501] Valley, S. L. Handbook of Geophysics and Space Enviroments, McGraw-Hill, New York (NY), 1965, 522p.
[6601] Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York (NY), 1966, 899p. [6801] Parker, C. A. Photoluminescence of solutions, Elsevier: Amsterdam (The Nether-lands), 1968, p. 186-191.
[8001] Beck, R.; Englisch, W.; Guers, K. in Table of Laser Lines in Gases and Vapors, Springer-Verlag: New York (NY), 3rd edition, 1980 (Springer Series in Optical Sci-ences, vol. 2), 132p.
[8101] Laporta, P.; Zaraga, F. Appl. Opt.1981, 20, 2946-2950.
[8201] Rabek, J. F. Experimental Methods in Photochemistry and Photophysics; Wiley: Chichester (UK), 1982, 1098p.
[8901] Scaiano, J. C. (ed) Handbook of Organic Photochemistry; CRC Press: Boca Raton, (FL), 1989, vol. 1, 451p.
[9301] Van Driel, H. M.; Mak, G. Can. J. Phys. 1993, 71, 47-58.
[9701] Andrews, D. L. (ed) Lasers in Chemistry: Third Completely Revised and Enlarged Edition; Springer-Verlag: Berlin (Germany), 1997, 212p.
[9801] Anmann, M.-C.; Buus, J. Tunable Laser Diodes, Artech House: Boston (Ma), 1998, 289.
[9901] Hou, X.; Zhou, J. X.; Yang, K. X.; Stchur, P.; Michel, R. G. Adv. Atomic Spectrosc.
1999, 5, 99-143.
[0401] Kuhn, H. J.; Braslavsky, S. E.; Schmidt, R. Pure Appl. Chem. 2004, 76, 2105-2146. [0501] Braslawsky, S. E.; Houk, K. N.; Verhoeven, J. W. Glossary of Terms Used in Pho-tochemistry, 3rd Ed.; IUPAC Commission, Pure Appl. Chem., in press.。