BHAUMIK, FLETCHER, YUGENT, LEE, HIGA,TELK,AND WEINBERG
1490
Laser Emission from a Europium Benzoylacetonate Alcohol Solution'
by M. L. Bhaumik, P. C. Fletcher, L. J. Nugent, S. M. Lee, S. Higa, C. L. Telk, and M. Weinberg Electro-optical Systems, Inc., Pasadena, California
(Received January 9, 1964)
Laser emission a t 6130 8. from a cooled alcohol solution of europium benzoylacetonate has been verified. The exact chemical constitution of the niicrocrystalline solute has been determined to be 1 mole of europium with 3 moles of benzoylacetonate, 1 mole of benzoylacetone, and 1 mole of piperidine. These units are combined to form a microcrystalline chemical adduct with unknown structure. Details of the chemical preparation, the melting point, and some of the characteristic emission spectra from the alcohol solution and the microcrystalline state are presented. Finally, a discussion is given of the laser threshold condition, and a reason why some europium chelates do not yield laser emission is proposed.
Introduction Observation of stimulated emission of 6130 8. from an alcohol solution of europiuni benzoylacetonate was reported by Lempicki and Sainelson2 and by Schimit~ c h e k . The ~ former authors designate the europium chelate as the three-ligand compound Eu(B), but give no method of preparation or melting point; the latter author gives no indication of the chemical composition and no melting point but states that the compound was prepared by the piperidine method of Whan and C r ~ s b y . However, ~ Whan and Crosby did not report the use of the pipeiidine method for the preparation of europium benzoylacetonate ; they report this method for the preparation of the rare earth dibenzoylniethides and give another method, in aqueous solution, for the preparation of the rare earth benzoylacetonates. This means that the reported europium benzoylacetoriate laser emission has been obtained froin a chelate which is essentially uncharacterized. The purpose of this paper is to report the preparation, the melting point, the chemical coniposition of the solute, and the wave lengths of some of the emission lines for the europium benzoylacetonate which yields laser emission. The importance of the characterization of the europium benzoylacetonate laser material became apparent when samples of widely varying melting point, degree of hydration, coordination number, chemical composition, and emission wave lengths were obtained, depending upon the method of preparation and the technique of purification employed. For example, recrystallizaT h e Journal of Physical Chemistry
tion from solution gave products varying in melting point from 90 to B O 0 , depending upon the solvent, the fraction taken, the solution temperature, and the time the solution stands before precipitation. Of more consequence is the fact that laser emission has been observed only from a particular europium benzoylacetonate product obtained by a modification of Whan and Crosby's piperidine method. *
Preparation and Characterization Preparation of europium benzoylacetonate by the aqueous method described by Whan and Crosby4 and by Sacconi and ErcoW gives the dihydrate. This material showed no evidence for laser oscillation when repeatedly tested under various conditions of low temperature and concentration in alcohol solution. In the same paper Whan and Crosby describe another nonaqueous method, which we call the piperidine method, for the preparation of rare earth trisdibenzoylmethides. In the piperidine method the product is vacuum-dried at 125 to 130' in order to remove a fourth mole of chelating agent. However, when this method was applied in the synthesis of europium benzoylacetoiiate, the products were of varying melting (1) Work supported in part by the Rome Air Development Center under Contract A F 30(602)-2914. (2) A. Lempicki and H. Samelson, Phys. Letters, 4, 133 (1963). (3) E. J. Schimitschek, A p p l . Phys. Lettecs, 3 , 117 (1963). (4) R. E. Whan and G. A. Crosby, J . M o l . Spectry., 8 , 315 (1962). ( 6 ) L. Sacconi and R. Ercoli, Gazr. chim. {tal., 79, 731 (1949).
LASEREMISSION FROM
A
point and composition depending upon the time allowed for the high temperature drying; decomposition obviously occurs during the drying process. By eliminating the vacuum drying step in the piperidine method, a stoichiometric crystalline compound was obtained that yielded laser emission from alcohol solution. The results of chemical analysis, as shown in Table I, indicate that the empirical formula of the crystalline compound is EuB,HP, a new chemical adduct
Table I : Theoretical Percentages of Constituents for EuBJIP and Measured Percentages of the Constituents of the h e r Chelate
EuB,HP Theoretical Measured
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EUROPIUM BENZOYLACETONATE ALCOHOL SOLUTION
C
H
N
EurOa
61.2 61.5
5.5 5.6
1.6 1.7
19.9 19.9
of europium trisbenzoylacetonate with one molecule of henzoylacetone and one molecule of piperidine. The data presently available are insufficient to give any indication of the molecular structure of the crystalline compound or of the chemical composition in alcohol solution. So a question not yet settled is whether the hydrogen ion is attached to the nitrogen of the piperidine base and the four benzoylacetonate ions are coordinated to the europium ion; or, on the other hand, whether the hydrogen ion is attached to a henzoylacetonate ion, forming benzoylacetone, and the other three benzoylacetonate ions are coordinated to the europium ion, either alone or possibly together with the piperidine base. Whether this compound produces laser action depends upon its purity and that in turn depends upon small details of the preparation. The following technique consistently produced a chelate that lased : To a solution a t about 90' containing 2.5 g. (0.0155 mole) of benzoylacetone and 1.5 ml. (0.0155 mole) of piperidine in 25 ml. of absolute ethanol was added a warm filtered solution of 1 g. (0.00387 mole) of EuCla in a similar amount of absolute ethanol. The cooled reaction mixture was maintained at ambient temperature for 4 days and then filtered. The crystalline precipitate was washed with cold absolute alcohol and air-dried to give 1.81 g. of product (52% yield), m.p. 125.6-126.5'. The results of a test for chloride ion in the product were negative indicating that the pipcridine hydrochloride formed in the preparation is not a contaminant.
Luminescence and Laser Emission Characteristies Laser action was demonstrated using a test system slightly modified from that described by Schiniitschek.s The EuBIHP was dissolved in an ethanol-methanol (3: 1) solution in order to maintain a liquid state with sufficient chelate solubility a t temperatures as low as -150". The solution was tested in a confocal laser cell 5 cm. in length and 0.1 cm. in i.d. Adjustable pistons with silver mirrors of 1% transmission are inserted into the ends of the test cavity forming a confocal optical system. The solution in the test cavity is optically pumped with a Kenilite high-energy helical flashtube. The flash energy passes through a filter with a 1500 A. bandpass centered a t 3700 A,, and is delivered to the system in a time of the order of 1 msee. Low temperatures are maintained by passing precooled nitrogen gas through the system. The entire unit is enclosed in a magnesium oxide coated reflector in order to lower the pump power requirements. The test cell output signal passes through a neutral density filter, then through a 100-A. bandpass interference filter with maximum transmission a t 6130 A., then to a phototube, and finally it is displayed on an oscilloscope. Following ultraviolet pumping, a characteristic intense Eua+ red luminescence is observed at 6130 A, M alcohol solution of Euand at 6142 A. from a B,HP at -150". The output of the laser test cell showing the emission intensity us. time is displayed in Fig. 1. The test cell was punipcd a t 5000 joules to produce this display; threshold energy was 1800 joules; laser spiking patterns are clearly demonstrated. In order to substantiate the interpretation of this phenomenon as laser oscillation and to determine which
Figure 1. Spiking d europium I,enzr,ylxcetrmate laser, sweep 100 ssec./division.
Volume 08. Number 0 June. 1904
BHAUMIK, FLETCHER, NUGENT,LEE, HIGA,TELK,AND WEINBERG
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of the red lines yield laser emission, the spectrum of the test cell output, under the same conditions as for Fig. 1, was measured on a medium resolution Bausch and Lomb quartz spectr2graph. In Fig. 2 we show the 6130 and the 6142 A. emission lines measured during normal emission and during laser emission. For purpose of line width comparison, the fluorescence emission from a low-pressure neon discharge is also show?. The neon lines are instrument broadened to about 1 A. the 6100-8. region. During laser oscillation the
I
I
I
In addition to the spectroscopic measurements made on the solution, the emission spectra of the microcrystalline solid were measured in order to further characterize the sample. As in the case of the ethanolmethanol solution, the luminescence spectrum of the EuB4HP microcrystals consists of two strong lines in the 6100-8 region. These are well known6to arise from the transition 6Do+ 7Fz of Eua+. Two lines are observed because the degeneracy of the 'F2 state is removed by asymmetric ligand fihlds.' The measured wave lengths and line widths are presented in Table 11.
I
Table I1 : Strong Emission Spectrum of Microcrystalline EuBIHP in the 6130-A. Region
Asirr
Line width;
6125.1 6132,6 6123.6 6130.6
2.0 1.0 4.0 3.5
A.
A:
Temp., OK.
77 77 298 298
a These values contain no contribution from instrument broadening.
1
6130
6120
I 6140
I 6150
I
I
M60
6170
WAVELENGTH, (A)
Figure 2 . Spectral line narrowing due to simulated emission in the alcohol solution of the europium chelate. Curve 1 shows the normal emission, curve 2 shows the laser emission, and the dotted curve shows the emission from the neon standard. The width of the neon lines is a measure of the instrument line broadening.
6130-A. line is about 1 8. in width while the emission intensity of the 6142-8. line is so low that it is not observed. Clearly from the narrowing in width and the relative increase in intensity of the 6130-A. line, laser emission from the alcohol solution of europium chelate is established. When the cell was near room temperature, the laser phenomenon was not observed, probably because of a higher threshold arising from decreased quantum efficiency, decreased emission decay time, increased line width, and increased thermal population of the IF2 terminal state (-lo3 cm.-'). This is reasonable in view of the temperature dependence of the spectroscopic propertie! : the quantum efficiency for emission from the 6130-A. line is 10% at -150" and less than 0 .lye at 27"; the luminescence decay time is 440 pse,c. at -150" and 140 psec. at 27"; the line width is 7 A. a t -150" and 15-20 8. at 27". The Journal
of
Physical Chemistry
Threshold Consideration and Discussion Assuming a Gaussian line shape, the minimum inverted population per cc. required for the threshold condition is given bys
where N 2 and N1are the population per ml. of the upper and lower states, T is the lifetime for spontaneous emission, X is the wave length of the emission peak, Av is the emission line width, R is the reflection coefficient, I is the length of the cavity, g2 and g1 are the degeneracies of the initial and terminal states, and 4 is the fraction of excited ions which decay by emitting the desired radiation. Only cavity losses due to transmission through the mirrors are included in this equation; cavity losses due to scattering, absorption, and diffraction a t the mirrors can make the observed value of AN somewhat larger. The total quantum efficiency for the 6130- and 61428.lines in the laser solution was measured to be 0.3, and (6) E. V. Sayre and S. Freed, J . Chem. Phgs., 24, 1213 (1056). (7) C. J. Bnllhausen, "Introduction to Ligand Field Theory," McGraw-Hill Book Company, Inc., Xew York, N . Y., 1962. (8) 8 . L. Schawlow, Solid State J . , 2 1 (1961).
LASEREMISSION FROM
A
EUROPIUM BENZOYLACETONATE ALCOHOL SOLUTION
about '/a of the emission is in the 6130-A. line so (b is 0.1 in this case. Because the quantum efficiency is 30T0 instead of close to loo%, the spontaneous emission lifetime can be llonger than the measured fluorescence lifetime with (an upper limit of 3.3 times this value. The degeneracy 92 of the initial 5Dostate ia one, and from the observed fine structure of the transition the degeneracy g1 of the terminal ligand field state is either two or one with the latter most probable. For the 6130-A. transltion in the Eu3+ion, the terminat-. ing state is -1.03 cm.-l above the ground state so NL can be considered to be zero at -150°. Substituting values into the above equation, we calculate AN to be 5 X 10leto 17 X 10leper ml. depending on whether the lower or the upper limit is taken for the radiative lifetime. The minimum concentration of chelate molecules for which laser emission was observed was lo-$ 144 or 60 X 10le molecules/ml. Since the actual population inversion a t threshold must be less than the molecular concentration, an upper limit for the measured AN is 60 X 10l6molecules/ml., a factor of 3 to 12 greater than the ca,lculated value. The difference between the measured and calculated threshold is a measure of optical losses from sources other than transmission through the mirrors. Other important losses
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are mirror diffraction, refractive index gradients caused by thermal effects, or absorption from the initial laser level to higher energy levels. A comprehensive program has been undertaken to learn why only certain samples of europium benzoylacetonate show stimulated emission and why others do not. This study is expected to point out specifically the molecular structure and composition of these chelates in alcohol and in other solvents, and to establish the factors determining the stimulated emission processes. Threshold analysis on the basis of the luminescence characteristics has given no clue. An explanation is offered as follows: it is possible that absorption to a higher level from the emitting state occurs; this can compete with stimulated emission and thus eliminate or reduce laser action. Such an absorption may occur in those europium benzoylacetonate chelates showing no laser emission. The higher absorbing state may be the ligand singlet, some higher triplet of the ligand, or an ion absorption band.
Acknowledgment. The authors wish to thank N. A. El-Sayed, E. Schiniitschek, D. L. Fridge, E. Gonzalez, and S. George for their interest and helpful discussions.
Volume 68, Number 6 June, 1064
