US3657591A

US3657591A - High intensity far u.v. radiation source

Info

Publication number: US3657591A
Application number: US50203A
Authority: US
Inventors: Peter D Johnson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1970-06-26
Filing date: 1970-06-26
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18

Abstract

A far U.V. radiation source emitting high intensity of U.V. at wavelengths principally shorter than 2,000 A.U. includes an electric discharge in a low pressure of mercury vapor at a pressure 2x10 3 to 0.1 torr and krypton gas at a pressure of approximately 0.5-10 torr. During operation, lamp current is very high, in range of 0.5 to 40 ampere/cm.2, emitting principally at a wavelength of 1,942 A.U.

Description

United States Patent Johnson 154] HIGH INTENSITY FAR U.V.

RADIATION SOURCE [72] Inventor: Peter D. Johnson, Schenectady, NY.

[73] Assignee: General Electric Company- [22] Filedi June26, 1970 [21] Appl.-No.: 50.203

[52] US. Cl ...313/2 23, 313/112,:413/227 [5 1] Int. Cl. ..H01J 17/20 [58] Field of Search ..3l3/1 12, 223,224, 225,226, 313/227, 228, 229

I 5m 1 References Cited UNITED STATES PATENTS Re2l,l50 7/1939 Von Lepel .....313/227X n51 3,657,591 451 Apr. 18, 1972 2,034,572 3/1936 Found ..3l3/223 X 2,757,305 7/1956 Dziergwa ..'..313/112 3,042,829 7/ 1962 Humphreys ..3 1 3/223 X Primary Examiner-Alfred L. Brody Attorney-Paul A. Frank, John F. Ahern, Jerome C. Squillaro, Frank L. Neuhauser, Oscar B. Waddell. and Joseph B. Forman s71 ABSTRACT v A far U.V. radiation source emitting high intensity of U.V. at

wavelengths principally shorter than 2,000 A .U. includes an electric dischargein a low pressure of mercury vapor at a pressure 2x10 to 0.1 ton and krypton gas at a pressure of approximately 0.5-10 torr. During operation, lamp current is very high, in range of 0.5 to 40 ampere/cm", emitting principally at a wavelength of 1,942 A.U.

9 Claims, 3 Drawing Figures 1 IIIGH INTENSITY FAR U.V. RADIATION SOURCE The present invention relates to far ultraviolet light sources. More particularly, the invention relates to such sources which emit radiation of wavelengths having enhanced photochemical stimulation properties otherwise previously unobtainable at such high and useful intensities and efficiencies.

This application is related to my co-pending, concurrently filed applications, Ser. No. 50,106 and Ser. No. 50,105.

Electric lamps emitting ultraviolet radiation generally utilize a gaseous discharge utilizing mercury as the emitting species. In most prior art devices utilized for this purposes, the lamp parameters of low current (below 0.2 amperes per square centimeter) and low pressure of emitting species (below 1 torr), are such that the principal radiation supplied is above 2,300 A.U., primarily that of the 2,537 A.U. line which is so strong, under the parameters of the prior art, as to usually dominate such ultraviolet emission. I have found that, although the 2,537 A.U. ultraviolet wavelength emission is useful for many purposes, it is very inefficient in causing many photochemical reactions, as for example, crosslinking of polymers and breaking of polymeric bonds. Other U.V. emitting lamps, which emit shorter wavelength radiation, are operated at high pressure (several atmospheres) and high current, (above 1 ampere per square centimeter), but still only emit useful U.V. radiation at wavelength longer than 2,300 A.U., which radiation is not effective for many photochemical reactions, particularly the crosslinking of many polymers.

in my co-pending, concurrently filed application, Ser. No. 50,106, I have disclosed and claimed broadly new far U.V. emitting lamps which include the concept of operating an ionizable metal vapor-electric lamp-at very low pressures (less than 1 torr) and very high currents (greater than 0.5 ampers/cmF), resulting in the emission of high intensity far U.V. radiation at wavelengths of less than 2,000 A.U. Thus, for example, mercury vapor at a pressure of less than 0.75 a/cm. and currents of 0.5 to 25 amperes/cm. emits high intensity radiation at 1,849 and 1,942 A.U. in an ambient of helium, argon or neon.

Despite the advantages of such lamps, it is desirable to 0p timize the output of such short wavelength U.V. radiation in order to make photochemical processes as efficient as possible. 1

Accordingly, it is an object of the present invention to pro,- vide mercury vapor-electric discharge lamps having optimum far ultraviolet emission intensity.

Still another object of the invention is toprovide mercury vapor-electric lamps with a maximum emission at 1,942 A.U. wavelength.

Briefly stated, in accord with the present invention, I provide, in one embodiment thereof, an evacuable envelope containing a quantity of mercury sufficient to yield, under operating minimum bulb wall temperatures of 25 to 80 C, an optimum pressure of mercury vapor of approximately 2X10 to 0.1 torr, permitting the emission of high intensity of far ultraviolet radiation, largely at a wavelength of 1,942 A.U. The excitation of the emitting mercury is greatly enhanced by interactions with a partial pressure of krypton gas. The enhanced excitation results in greatly enhanced emission intensity.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the following detailed description, taken in connection with the appended drawing in which FIG. 1 is a vertical view, with parts broken away, of a lamp constructed in accord with the present invention and suitable for operationfor the production of high intensity photochemically-useful far ultraviolet radiation. 1 FIG. 2 is an alternative embodiment to the device of'FlG. l

specially adapted for operation under unidirectional current excitation.

FIG. 3 is a graph containing comparative curves for one wavelength emission, illustrating the increase in intensity, as a function of lamp current density, of a typical lamp constructed in accord with the present invention over the most nearly similar lamp.

FIG. 1 illustrates a simplified form of far ultravioletlamp in accord with the present invention. The lamp of FIG. 1 includes an evacuable envelope, represented generally at 10, which includes an ultraviolet transmissive central member 11 and a pair of enlarged electrode-containing

end members

12 and 13, respectively. Each of

end members

12 and 13 includes a cathode assembly 14 which in this instance is separately heated and is mounted between the inboard ends of a pair of inleads l5 and 16, respectively. Each of inleads l5 and 16 has connected, immediately adjacent the filament, and substantially parallel thereto, a pair of auxiliary anode members 17 and 18, respectively. These members, on alternate cycles of an alternating current voltage exciting the device underalternating current operation, serve as anode: members to sustain an electric discharge. The desired current and voltage to operate the lamp is supplied by a power supply means, capable of supplying the requirements of the lamp in operation and may take a number of forms, but which is, for example, illustrated generally as a saturable transformer represented by dotted line box 19. Transformer 19 includes a primary winding 20 and a voltage step-up secondary winding 21 having a pair of tapped secondary low voltage portions 22 and 23 which are connected across respective pairs of

electrode inleads

15 and 16 on either end of the lamp so as to provide alternating current heating of each of the filaments 14 by the voltage developed across the tapped secondary. This voltage is necessarily low, of the order of several volts, to cause external heating of the filament to sustain the thermionic emission of electrons to sustain an electric discharge between the filament and anode members at respective ends of the arc tube. The filaments are essentially similar to conventional fluorescent lamp filaments, although the design is not critical. The lamp is charged with a low pressure of krypton and a sufficient quantity 24 of mercury.

In operation, the lamp is started by the application of a line voltage, which may be of any desired voltage but which may conveniently be or 240 volts, to the primary of the excitation transfonner 19. Due to the external excitation of the filaments 14, the lamp is immediately operative. A quantity of vaporizable mercury is present as charge 24 within the lamp, as is a filling of a relatively low pressure, as for.example,

0.5-25 and preferably approximately 2-5 torr of krypton. Operationally,-the application of the initial voltage causes an electric discharge to be sustained by the krypton, which is immediately ionized thereby, which excited gas discharge maintains the bulb wall at a temperature consistent with the desired vapor pressure of mercury within the lamp envelope to cause the establishment of a sufficient ionizationthereof and transfer the conducting specie.

FIG. 2 illustrates another embodiment of the invention, functionally equivalent to the device of FIG. 1: but having certain structural modifications, illustrating the versatility with which structure embodying the invention may be constructed.

in FIG. 2, an evacuable envelope represented generally as 29, includes an ultraviolet transmissive central portion 30, having three serpentine curves and four substantially straight sections, shown for convenience: only. It should be ap preciated that the number of straight sections and thenumber of serpentine curves may be increased to any desired number and the total length of the discharge path may be made any desired length, generally being tailored to fit the size of the total light emitting area in order to suit the purpose to which the lamp is put, generally in the reactive photochemical processing 'of monomers and polymers, oftenin the form of of the discharge to mercury, as

pressure of mercury to permit clude a cathode assembly 33 and an anode assembly 34. Cathode assembly 33 may conveniently have the so-called Ml cathode structure, that is a dispenser type filament member 35 containing a single loop and fabricated from a mesh stocking container containing a particulate mass of a thermionic emitting substance, such as barium aluminate or lanthanum boride, for example. A cathode shield member 36 laterally surrounds the filament member 35 and contains an aperture 37 therein for the escape of electrons to sustain an electric discharge along the axis of the first substantially straight portion 38 of central region 30 of envelope 29. Anode assembly 34', in its simplest structure, contains a collector means which in this instance, is shown as a hollow cup 39 mounted upon an inlead 40 which passes through pinch 41 in the envelope end, just as filament 35 is mounted upon, and electrically connected between, inleads 42 and 43, which pass through pinch 44 in envelope end 31. It is desirable that cathodes and anodes of these general types, respectively, be used under direct current excitation in order to maintain the high rate of current conduction in the steady state as may be required, although it is to be appreciated that for direct current excitation, any suitable cathode and anode structure which are able to maintain current densities of up to 100 amperes per square centimeter but preferably up to 25 amperes per square centimeter are suitable.

Similarly, although a particular type of electrode structure is shown in the device of FIG. 1 for the maintenance of the high current density characteristic of lamps in accord with the A.C. excited embodiment of the present invention, it should be appreciated that any similar electrode structure which is capable of maintaining the aforementioned current densities under alternating current excitation is suitable. Alternatively, for alternating current operation, a lamp may be fabricated without electrodes and excitation thereof may be accomplished in accord with the structural arrangement described in application Ser. No. 653,749, filed July 17, 1967, now U.S. Pat. No. 3,500,118, in the name of J. M. Anderson, and assigned to the present assignee. In such an arrangement, a closed loop constitutes the discharge path within an hermetically sealed envelope containing an appropriate ionizable fill. A radio frequency oscillator is connected to a primary winding which is coupled to the closed loop through a high radiofrequency permeable ferrite core. The secondary of the excitation transformer is the discharge path through the closed loop of the lamp which passes through a portion of the core and is excited thereby. Such systems operate ideally at frequencies of 500 kilocycles and above an are suitable for the production of high current density at low pressure, in accord with the present invention, wholly independent of electrode phenomena.

In the embodimentof FIGS. 1 and 2, krypton of a pressure of approximately 0.5-25 torr and preferably approximately 2-5 torr is contained within the envelopes. Similarly, a suitable quantity of mercury is included within the envelope and is adaptable, upon heating by an initial discharge established within the krypton to be vaporized and ionized such as to cause the discharge essentially to become a mercury vapor discharge at low pressure and high current density for efficient emission of the characteristic 1,942 A.U. mercury ion line.

As is pointed out in my aforementioned co-pending application, Ser. No. 50,106, whereas the prior art workers have been unable to produce any photochemically useful intensity of either the 1,849 A.U. or the 1,942 A.U. resonance line of'mercury under previous, normal conditions of operation of vapor discharge lamps, namely that of relatively high pressures, I am able in accord with the teachings of my above-identified copending application,- by controlling the mercury pressure within the lamp to be within the range of approximately 3X10 4 to 0.75 torr, while simultaneously maintaining the currents through the lamp at a current density in the range of approximately 0.5 to 25 amperes per square centimeters, to obtain a high degree ofjntensity of the 1,849 A.U. line and the 1,942 A.U. resonance line of mercury. The intensity of the 1,942

A.U. line alone, at high current densities far exceeds the intensity of emission of the conventional 2,537 A.U. radiation, which is the only usefully efficient far ultraviolet mercury radiation by lamps of the prior art.

The low pressures of mercury within lamps of this type is maintained primarily by maintaining control of the coldest portion of the lamp envelope wall, generally in the region of the cathode and anode electrodes. As'hereinbefore utilized, the term bulb wall temperature," will identify the minimum temperature at which any portion of the interior of the bulb wall exists during steady state operation. This temperature is utilized as a frame of reference, because it effectively controls the pressure of the mercury vapor within the lamp by virtue of the fact that the pressure of mercury vapor within the lamp is controlled by the degree of condensation of the vapor at the coldest portion of the bulb wall. In accord with the present invention, in order to optimize the 1,942 A.U. emission from the lamps thereof, I maintain a bulb wall temperature within the range of approximately 15 to 100 C, and a preferred range of from approximately 25 to C, for the optimum emission of 1,942 A.U. resonance mercury ion radiation. The preferred temperature yields an operating pressure of mercury within the envelope within the range of approximately 2X10 torr to 0.1 torr, all of which ranges are substantially broader than the comparable ranges for lamps in accord with my aforementioned co-pending application.

The current density within the lamp is maintained at the desired range by appropriately adjusting the total current through the discharge and the diameter of the narrow, U.V. transmissive portion of the lamp envelope. The total current is controlled by external impedances, and is adjusted to obtain maximum output from the radiating spectroscopic states of the radiant species.

In general, for alternating current operation, lamps in accord with the present invention may readily operate at a voltage from 20 to volts A.C. at a current density of 10 amperes, per cm. although operation at current densities up to 25 amperes per square centimeter is quite useful and lamps have been operated at current densities of as high as 100 amperes per square centimeter. A typical lamp configuration for the attainment of such operation, namely, at a current density of approximately 10 amperes per square centimeter and a pressure of approximately 3 torr, may readily be obtained within a lamp envelope having an interior diameter within the ultraviolet transmissive region of approximately 1 centimeter and a length of approximately 50 centimeters with an applied voltage of 50 volts.

In the lamps of my aforementioned co-pending application, Ser. No. 50,106, I used argon, helium or neon, all inert, nonreactive gases, as buffer or fill gases. This is in accord with general1y-accepted practice in vaporelectric lamps. Since the buffer gas has little function other than to form an initial discharge to heat the mercury and does not otherwise interact therewith, the lighter, more plentiful and less expensive gases helium, argon and neon are used. In accord with the present invention, 1 have introduced a new concept, namely that of energy exchange between the buffer gas and the mercury within the volume of the light-emitting region. 1 postulate two energy exchange reactions.

where Hg* and Kr* are metastable excited states of mercury and krypton, respectively. I i

It is believed that mercury is ionized by ionization of mercury by the metastable krypton according to equation (1). This is possible because krypton, unlike helium, argon or neon, has three metastable states, the P at 9.90 eV, the P at 10.57 eV and the P at 10.62 eV, all of which may energize mercury, whose ionization potential is 10.43 eV. On the other hand, the lowest excited states of the common noble gases helium, neon and argon are respectively 19.8 eV 5,); 16.60 eV( P and 11.55 eV( P none of which may energize mercury to an ionized state.

The forward direction of equation l is favored in the posi- Finally, the use of krypton with mercury at high currents and low pressures in accord with the present invention, makes the range of mercury pressures over which the mechanism of operation is possibly wider, and hence increases the bulb wall temperature range. This is so because the reverse direction of equation (1) maintains lower l-lg concentration without undue dissipation of energy, which reduces the effectiveness of'the first portion of equation (2). Thus, the presence of krypton makes possible the attainment of high intensity, high efficiency output at higher mercury pressures which correspond to higher wall temperatures than when lighter, more common noble gases are utilized, as a buffer gas, and do not enter into the excitation mechanism of the mercury.

Thus, for example, the operating pressure of mercury within which'useful light may be obtained in accord with the krypton and mercury fill of lamps of the present invention, is from approximately 2 l0' to 0.1 torr, corresponding to a temperature range of approximately to 80 C, a highly practical and-easily attainable operating range.

Although in accord with the present invention, krypton excited states are preferably used to enhance the ionization of the exciting mercury, 1 am able to obtain similar results utilizing a like fill of xenon gas. Xenon has metastable states identified by Paschen notations 2p at 10.95 eV; 2p at 11.0 eV; 2p at 11.0; 2p at 11.1 eV which may ionize mercury atoms to an energized state.

FIG. 3 of the drawing illustrates a typical plot of lamp intensity, in arbitrary units, plotted as a function of lamp current, in amperes per square centimeter of a lamp constructed in accord with my aforementioned co-pending application, Ser. No. 50,106, having a mercury-argon fill (Curve A) and a lamp constructed according to the present invention having a krypton-mercury fill (Curve B), while Curve A is a straight line havinga slope of 1.7, Curve B has a slope of 2.0. The logarithmic plots indicate the exponential power of the slope of the curves. The emission of Curve B at current densities above approximately 5 amperes/cm. is from 2 to 3 times brighter than Curve A lamps. The principal limiting factor upon the ultraviolet intensity obtained from lamps of the present invention is the saturation of the emission of the excited mercury. As a practical matter, for most modes of operation, a useful maximum of approximately 40 amperes per square centimeter is obtainable.

Lamps operated in accord with the present invention have very unusually high intensity of emission of the 1,942 A.U. lines. Additionally, some emission, at useful intensity of the 1,849 A.U. line is present. These wavelengths, heretofore emitted only trivially by lamps of the prior art, are emitted with high efficiencies. Another advantage of lamps in accord with the present invention is that with krypton present, the mercury 1,650 A.U. line, not otherwise visible, is pronounced, and when the lamp envelope is transparent to this line, or has a section thereof which is transparent thereto, this radiation is also available for photochemical purposes. A sapphire section of the envelope is an excellent window for 1,650 A.U. emisslon.

l have found that the scientific characterization of the lamp in accord with the invention has been that of a high electron temperature within the positive column of the discharge and that'the discharge is distributed over the entire positive column and is not a mere cathode phenomenon. This is particularly evidenced by the fact that the lamps in accord with' the present invention may be operated in electrodeless environments. With this high electron temperature, I find that a high percentage of the excited mercury specie exists in the atomic l, spectroscopic state and the ionic P and P spectroscopic states.

In accord with-another feature of the present invention, I provide a sufficient quantity of mercury to the lamp envelope so that there is always an excess of mercury within the lamp and the vicinity of the coldest portion of the bulb wall. This provides for cleaning up of the mercury specie by deposition of metallic mercury upon lamp parts or on the lamp envelope wall without decreasing the pressure below the desired predetermined pressure at which the lamp is to operate. As is mentioned hereinbefore, the lamps of the present invention are governed according to the interior diameter of the bulb wall in order that the current density for any given current may be controlled, although this control is not the exclusive factor. In order to maintain the current (and hence the predetermined current density) within the appropriate range, the voltage source is chosen to have an internal limiting impedance (or an external impedance is supplied) so that the current is limited, as for example, by saturation of a saturable transformer. The operating voltage is that required by the discharge path size and shape. Thus, for example, a given discharge of mercury at 0.01 torr may require an voltage of 1 volt per cm. in krypton or xenon at 3 torr ambient. A 50 cm. long discharge tube of a diameter, about 1 cm., requires a voltage of approximately 50 volts. Further choices of operating voltages are well within the purview of one skilled in the art. Generally, however, the lamps of the invention operate on a voltage which is generally of the order of 20 to volts, with an appropriateimpedance, and the interior bulb wall diameter varies over a range of approximately 3 to 40 millimeters, but is preferably maintained within the range of 5 to 25 millimeters, for maximum intensity and efficiency of U.V. light output. The length of the ultraviolet transmissive portion of the lamp may be any value above about 10 centimeters with no substantial upward limit, the upward limit being substantially governed by the configuration to which the lamp must conform for operational purposes.

Lamps constructed in accord with the present invention are of great utility in stimulating photochemical reactions. More particularly, the photopolymerization of thin monomeric films in the production of photoresist like substance is one very useful purpose to which lamps in accord with this invention may be placed. In particular, a very substantial use has been found in the photopolymerization of hexachlorobutadiene. Photopolymerization of hexachlorobutadiene with ultraviolet radiation is described, from the chemical viewpoint, in greater detail in the application of A. N. Wright, Ser. No. 648,132, filed Feb. 23, 1967, now US. Pat. No. 3,522,076. Other uses for lamps of the present invention are in the radiation of other photoresists and the stimulation of various chemical reactions such as photosynthesis. Previously, commercially available ultraviolet light sources have been utilized for this purpose, but all such low pressure, low current lamps emit predominantly the 2,537 A.U. mercury line. Similarly, high pressure, high current mercury vapor lamps have been used to emit shorter wavelength U.V. radiation, but still at wavelengths of 2,300 A.U. or longer. Such wavelengths are not as effective for many chemical reactions which are selectively responsive to effective radiation wavelengths lower than 2,000 A.U.

By the foregoing, l have described new and improved far ultraviolet emitting vapor discharge lamps selectively emitting sealed ultraviolet radiation of the 1,650 A.U. and 1,942 A.U. mercury lines which have high efficiencies in excess of 30 percent and higher luminous intensity in the far ultraviolet and which are selectively useful for stimulating photochemical reactions.

While the invention has been described herein with respect to certain specific examples on preferred embodiments thereof, many modifications and changes will readily occur to 1 What I claim as new and desire to secure by Letters Patent of the United States is:

l. A far ultraviolet emitting lamp comprising a. an evacuable bulb having an ultraviolet emissive portion in which at least a portion of the bulb is transmissive of ultraviolet radiation of wavelengths shorter than 2,000 A.U.; b. a filling within said envelope including b,. a partial pressure of a material selected from the group consisting of krypton and xenon noble gas with the range of approximately 0.5 to 25 torr, and

b a quantity of mercury sufficient, under lamp operating conditions wherein the coldest portion of the bulb interior is maintained at a temperature of approximately 25 to 80 C, to maintain within said bulb a partial pressure of mercury of approximately 2X10 to 01 torr;

c. means for coupling to said filling a voltage sufficient to ionize said krypton and establish an electric discharge therein, said discharge comprising krypton and mercury;

d. said discharge comprising ionized mercury which when excited by current densities of approximately 0.5 to 100 amperes/cmf emits ultraviolet radiation at wavelengths shorter than 2,000 A.U. as the principal emission thereof.

2. The lamp of claim 1 wherein said current density is at least 10 amperes/cm. and said radiation is principally at 1,942 A.U.

3. The lamp of claim 1 wherein said current density is in excess of 10 amperes/cm. and a significant proportion of said ultraviolet radiation is at 1,650 A.U.

4. The lamp of claim 3 wherein at least a portion of said bulb wall in the region of the discharge positive column is sapphire.

5. The lamp of claim 1 wherein said noble gas is present within said bulb at a pressure of approximately 2-4 torr.

6. The lamp of claim 1 and further including a pair of indirectly heated electrodes for sustaining an alternating current discharge within said bulb.

7. The lamp of claim 1 and further including a pair of electrodes, one of which is a thermionically emissive cathode for sustaining a direct current discharge within said bulb.

8. The lamp of claim 1 wherein said means for coupling a voltage with said filling is located externally of said bulb and is inductively coupled therethrough.

9. The lamp of claim 1 wherein said noble gas is krypton.

Claims

1. A far ultraviolet emitting lamp comprising a. an evacuable bulb having an ultraviolet emissive portion in which at least a portion oF the bulb is transmissive of ultraviolet radiation of wavelengths shorter than 2,000 A.U.; b. a filling within said envelope including b1. a partial pressure of a material selected from the group consisting of krypton and xenon noble gas with the range of approximately 0.5 to 25 torr, and b2. a quantity of mercury sufficient, under lamp operating conditions wherein the coldest portion of the lamp bulb interior is maintained at a temperature of approximately 25* to 80* C, to maintain within said bulb a partial pressure of mercury of approximately 2 X 10 3 to 0.1 torr; c. means for coupling to said filling a voltage sufficient to ionize said krypton and establish an electric discharge therein, said discharge comprising krypton and mercury; d. said discharge comprising ionized mercury which when excited by current densities of approximately 0.5 to 100 amperes/cm.2 emits ultraviolet radiation at wavelengths shorter than 2,000 A.U. as the principal emission thereof.

2. The lamp of claim 1 wherein said current density is at least 10 amperes/cm.2 and said radiation is principally at 1, 942 A.U.

3. The lamp of claim 1 wherein said current density is in excess of 10 amperes/cm.2 and a significant proportion of said ultraviolet radiation is at 1,650 A.U.

9. The lamp of claim 1 wherein said noble gas is krypton.