1785
APROTIC SOLVENTSFOR Nda+LIQUIDLASERSYSTEMS
Comparison of Aprotic Solvents for Nd3 Liquid Laser Systems: +
Selenium Oxychloride and Phosphorus Oxychloride' by C. Brecher and K. W. French The Bayside Laboratory, Research Center of General Telephone & Electronics Laboratories Inc., Bayside, New Yorh 11660 (Received December 9, 1968)
The properties of SeOClz and POCls as aprotic solvents for Nda+ liquid lasers are compared. Details are given concerning preparative procedures, absorption and emission spectroscopy, energy levels and radiative lifetimes, and laser performance. The two solvents are shown to be equally suitable for use in high-energy liquid lasers.
Introduction Aprotic solvents, because of their freedom from the high-energy vibrations which efficiently degrade energy from excited ions, have received considerable attention as host media in liquid laser application^.^-^ The first such material in which laser action was achieved contained selenium oxychloride (SeOCL) as its major component; this liquid, with a dielectric constant of 46, is an excellent medium for the dissolution of ionic salts (such as those of Nd3+), particularly when acidified by the addition of tin tetrachloride (SnCl4) or other similar aprotic acids. Subsequently, another system based on phosphorus oxychloride (POC13) was developed; this solvent, with a dielectric constant of 14, is a less wellsuited host for ionic salts but is more attractive for liquid laser uses because it is far less toxic and corrosive than SeOClz. This paper compares the chemical and spectroscopic properties of these two systems and relates them to the laser characteristics.
Experimenta 1 Section The similarities and differences in the chemical properties of the two liquid laser systems were reflected in their preparative procedures. With SeOC12, the laser solution was prepared by directly dissolving neodymium oxide in anhydrous SeOC12 which had been acidified with SnC4 (in a volume ratio of approximately 5 : l), a reaction which can be described by Nd20a
+ 3Sricl4 + 3SeOCl2 2Nda+ + 3SnCle2- + 3Se02 ----f
(1)
To remove hydrogen-containing contaminants (such as H2O or HC1) which are the principal sources of nonradiative quenchingjBthe resulting mixture was distilled at a controlled pressure of 40 mm until a constant boiling point of about 90" (pure SeOC12)was reached and about one-third of the total solution had been removed. This procedure differs from that reported earlier' in that the lower temperature of distillation prevented the discoloration of the solution by SezClz,ob-
viating the need for bleaching. At this point, analysis of the remainder for tin showed a solution which contained only about half of the stoichiometric quality of SnCld as defined in eq 1. The finished laser solution was then prepared by dilution with completely anhydrous SnCla and SeOClnto reach the desired concentration of Nd3+and the desired acidity. For this investigation, a solution was prepared having an Nd3+ concentration of 0.3 M and an SnCL concentration of 0.1 M in excess of stoichiometric concentration. This solution had a fluorescence decay time of 260 Fsec. With POcl3, the dissolution of Nd203can nominally be described by the same sort of equation as for SeOClz NdpO3
+ ~ S I ~ +C I6POC13 ~
--t
m d 3 + -t 3SnC162- 4- 3PZO3Cl4 (2) However, because of the much lower solubility of rare earth salts in POCll (even when acidified with SnC14 in the same volume ratio as with SeOCls), concentrations of Nd3+desired for laser solutions could not be achieved and maintained in the pure, completely anhydrous solvents. If, on the other hand, amounts of water were added to the acidified POCISin a molar ratio of about 1: 10, the solubility of the Nd3+ was dramatically increased, enabling concentrations greater than 2 M to be readily achieved. The reactions of water with POC13 have been found8 to yield a large number of compounds (1) T h i s research was partially supported by Project Defender under the joint sponsorship of the Advanced Research Projects Agency, the Office of Naval Research, and the Department of Defense, under Contract No. N00014-68-C-0110. (2) (a) A. Heller, Appl. Phys. Letters, 9, 106 (1966); A. Lempicki and A. Heller, ibid., 9, 108 (1966); (b) T. M. Shepherd, Nature, 216, 1200 (1967). (3) D. Kat0 and K. Shimoda, Japan. J. Appl. Phys., 7 , 648 (1968). (4) V. P. Belan, V. V. Grigoryants, and AM.E. Zhabotinski, IEEE Conference on Laser Engineering and Applications, Washington, D. C., 1967. (6) N. Blumenthal, C. B. Ellis, and D. Grafstein, J . Chem. Phys., 48, 6726 (1968). (6) A. Heller, J . Am. Chem. SOC.,88, 2068 (1966). (7) A. Heller, ibid., 90,3711 (1968).
Volume 73, Number 6 June 1969
1786
of phosphorus with oxygen and chlorine, of which PZ03CL and HPOzCl2 are prime examples. It was believed that one or more of these products formed some sort of complex which greatly increased the solubility of Nda+ in the POCls-SnC14 solvent. It then became necessary to return the solution to an aprotic state, without at the same time removing the desired products, either by reversing the reaction which produced them or by destroying them in a further reaction. This was achieved by boiling off (at atmospheric pressure; final boiling point 116') a sufficient fraction of the solution (in this case about two-thirds) to remove all of the residual HC1 and HzO. Finally, the remaining solution was diluted with anhydrous POC&and SnC14 to the desired concentrations. A POCls solution having an Kd3+ concentration of 0.3 1%' (the same as for the SeOC1, solution) was prepared in this manner; its fluorescence decay time was 245 bsec.
Discussion
As hoats for the Nda+ ion, phosphorus oxychloride and selenium oxychloride have many gross similarities. Both are polar, strongly coordinating solvents; both are aprotic solvents, with their highest fundamental at least a factor of 3 lower than that of water; and both have proven highly satisfactory media for liquid laser applications. Because of this last feature, it is pertinent here to discuss the material properties of the two systems and the manner in which these properties affect the laser performance, Absorption Spectra. The absorption spectra of the two systems at an Nda+ concentration of 0.3 1%' were measured with a Cary 14 spectrophotometer and are shown in Figure 1. In general, the transitions in POCla are shifted somewhat to shorter wavelengths compared with the corresponding ones in SeOC12. In addition, many of these transitions, including both the major resonance pump band (-5800 A) and the 4F8/2-41~/2 transition (-8700 A), are considerably less intense. This has the cumulative effect of decreasing the integrated absorptionsg Sa@) dh for Nds+from 4.9 X in SeOClz to 3.5 X in POClB, over the same spectral region (4000-10,000 8). The greater absorption in SeOClz does not, however, imply greater efficiency of optical pumping, since the major pump bands are sufficiently intense to absorb a large part of the energy near the surface and thus to prevent it from reaching the center of the laser cell. A quantity more closely related to the actual pumping rate a t a distance r from the surface is sa(X)e-OCr di;g for a typical value of 1' = 5 mm, these integrals are approximately 1.09 X lop6in SeOClz and 1.07 X in Poc13 over the same spectral region, hardly a significant difference. More important, however, is the fact that thed solvent transmission cutoff is 3200 8 in P0Cl3, 900 A lower than in SeOC12,which exposes another intense absorption band of Nd3+ for use in pumping. This increases the total T h e Journal of Physical Chemistry
C. BRECHER AND K, W. FRENCH
WAVELENGTH (A)
Figure 1. Absorption spectrum of 0.3 M Nda+ laser solutions at 300'K: (a) SeOClQ; (b) POCla.
.
5 15-
.-
c
C L L
.-E
-
.LI 0 10-
+
8600
8800
9000 io.400 WAVELENGTH (A)
10,600 la 300
Figure 2. Emission spectrum of 0.3 M Nd*+ laser solutions at 300'K: (a) SeOCL; (b) POCla.
value of fa@) dh for POCla to 3.8 X 10-6 and of Jct(h)e-"' dX to 1.18 X lop6,somewhat more favorable than for the SeOClz system. Emission Spectra and Energy Levels. The emissions from the two systems, excited by a Hanovia 538C-1 xenon arc lamp with Corning 3-69 and 4-97 filters, were measured with a Jarrell-Ash 0.5-m Ebert monochromator and an ITT FW 118 photomultiplier. The emission spectra at 25' are shown in Figure 2. Aside from a slight general shift of the emissions to shorter wavelengths, the pertinent point to observe is that the proportion of the emission in the 1.06-p region is greater in POC1, than in SeOC12. The structures of the emissions are somewhat different, but the breadth and resultant incomplete resolution preclude any firm assignments of the components of the transitions involved. However, when the temperature is lowered to -lOO"K, both solutions become rigid glasses, remaining clear and uncracked. At this temperature most of the comtransition are clearly resolved ponents of the 4F8,2-419,2 in the emission spectra (Figure 3), and the shorter wavelength components appear in the absorption spectra as (8) J. R. Van Wazer, "Phosphorus and Its Compounds," Interscience Publishers, New York, N. Y., 1961. (9) With )A(. in cm-1 and A in cm, these integrals are dimensionless. The wavelength distribution of incident photons, which for our flashlamps is nearly flat over the spectral region involved, has been omitted from the integrals.
17S7
APROT~C SOLVENTS FOR Nd3+ LIMU~D LASERSYSTEMS Table I : Components of the 4Fa/,-41~/,Transition of Nd*+ (0.3 M ) in SeOCl2 and POC1,
> o l
--
c
n
lenFth, A
.
9200
WAVELENGTH
(A)
Figure 3. Emission spectrum of 0.3 M Nda+ laser solutions a t 100'K: (a) SeOCll; (b) POC11.
c
Energy, cm-1
Assignmenta
lenfth,
En e w ,
A
cm-l
8668 8705 8738 8778
11,537 11,488 11,444 11,393
8628 8679 8718 8768
11,590 11,522 11,471 11,405
8801 8865 8905 8950 -8990
11,362 11,280 11,230 11,173 11,123
b-1 a- 1 b-2 a-2 b-3 a-3 b-4 a-4 b-5 a-5
8805
11,357
8889 8960
11,250 11,162
40.
a
.
W P
a Components of the 410/2 level are numbered consecutively from the ground state (1); for the &Fa/,level, a denotes the lower component and b the higher. These assignments yield the respective following values (cm-1) of energy levels 1, 2, 3, 4, 5, a, lb: for SeOClz: 0, 94, 125, 245, 352, 11,487, 11,537; for POCIS: 0, 118, 165, 272, 428, 11,522, 11,589.
-
2W v
7
through the ratio of the respective emissions. The results of applying this method (see ref 10 for details) to the SeOC12and POCl, systenis (at 0.3 41 Yd3+concentrations) are listed in Table 11.
I I
5a
Po&-
Wave-
Y)
2
----
SeOCll----7
Wave-
c
Table 11: Spectroscopic Properties 0.3 IK Nd3+ Laser Solutions (300'K) SeOClz'
20.
0.
' 8500
8700 WAVELENGTH
-
8900
(i)
Figure 4. Absorption spectrum of 0.3 M N d 2 +laser solutions at 100°K: (a) SeOClZ; (b) POCL.
well (Figure 4). This information, plus measurements made at intermediate temperatures, enables an assignment to be made of the various levels of both the "ground" 41e/2 multiplet and the upper 4F912multiplet. Unfortunately, the emission to the 411112state (the terminal state of the 1.06-p laser transition) is not sufficiently resolvable even at the lower temperatures to enable firm assignments of the components of that state. The data and those assignments that can be made are summarised in Table I. Tyansition Cross Section and Radiative Lijetimes. The absorption cross section for the laser transition is one of the important characteristics of laser systems. A simple methodlo for measuring this parameter in Nd3+ systems involves directly determining the oscillator strength of the resonance transition through absorption measurements and relating this to the laser transition
O d l a t o r Btrength in resonance transitiona Radiative lifetime of resonance transition,b Msec Absorption cross section of transition, cm2 Radiative lifetime of laser transition, psec Calculated fluorescence decay time, psec Observed fluorescence decay time (highest value), psec
POClS
7.8 X lom6 5 . 2 X 10-8 1780
2890
7 . 8 X 10-zo 9 . 6 X 10-zO 600
680
280
310
280
260
'Because of small changes in the identification of energy levels, the values for SeOCla differ slightly from those reported earlier.'O 'This transition involves oltly component 1 of the 9s/, level.
With regard to these results, two points are particularly significant. First, the 4Fa,2-418,2resonance transition is less intense in the POCI:, system than in the SeOClz system, while the emission in the 1.06-p region of the 4F8,2--4111/2 laser transition (normalized for equal pumping) is actually somewhat greater in the (10) H. Samelson, A. Heller, and C. Brecher, J . Opt, SOC.Am., 5 8 , 1054 (1968). Volume 73, Xumber 6 June 1969
1788
C. BRECHER AND K. W, FRENCH
60.3N
4000
3500
3000 FREQUENCY (ern-')
2500
NkPOCI,-SnCI4,
,
1
2000
Figure 5. Infrared absorption spectrum of SeOClt (0.3 M Nda+) laser solutions a t 300'K: (a) contaminated with HaO; (b) anhydrous.
KILOJOULES I N
Figure 7. Energy output from 0.3 M Nda+ laser solutions: 0, SeOClf; A, POCl3.
E l-
-
4000
3500
3000 FREQUENCY (ern-')
2500
2000
Figure 6. Infrared absorption spectrum of POCla (0.3 M (a) contaminated with HSO;
Nd3+) laser solutions at 300'K: (b) anhydrous.
POC13 system, Thus a considerably greater proporstate is emitted tion of the total energy reaching the 4F3/2 in the desired spectral region, while the over-all transition probability from this state is decreased, making possible (in theory at least) a more efficient laser system. On the other hand, a contradictory behavior is observed in the over-all fluorescence decay times. In SeOC12, the measured decay time is nearly identical with that calculated from spectroscopic considerations, implying an almost total absence of nonradiative losses from the emitting statc and hence a quantum efficiency close to unity, In POCl,, however, the measured decay time falls some 20% below the calculated value, implying the presence of some nonradiative loss. It is known that the major cause of such loss in these systems is an interaction between the ion and high-energy vibrations in the host medium, such as might be introduced by small amounts of hydrogen-containing impurities. The infrared spectra (Figures 5 and 6) show that The Journal of Physical Chemistry
neither of the solutions exhibits significant amounts of such contamination; however, the rather intense absorption in the region of the first overtone of the P-0 stretching vibration (compared with the extremely weak second overtone of the Se -0 vibration) suggests vibrational coupling through this transition as a possible source of nonradiative loss in the POCla system. In any event, the points mentioned emphasize the sensitivity of the various transition probabilities of the Nd3+ion to chemical interactions with the host medium. Laser Characteristics. Laser experiments on the two liquid systems were conducted under as nearly the same conditions as possible. A Pyrex cell 6 in. long with a bore diameter of 0.65 in, was used, A cell with demountable end windows" was chosen because of the importance of keeping the interior faces of the cell parallel to each other in order to minimize the effect of the refractive index mismatch between the cell materials and the solutions. The laser cavity was formed by external dielectric mirrors (one with 99.9% reflectance, the other 57y0)parallel to the cell window faces. The cell was filled with each of the two solutions and flashed in a close-coupled arrangement using three xenon flash lamps at input energies from zero to 640 J. Output energy measurements were made with a TRG 107 calibrated thermocouple detector and microvoltmeter. The results obtained are illustrated in Figure 7. It is seen that the slope efficiency for the POC13 system is 1.6%, almost 50y0 higher than for SeOClz. This is SO despite the fact that the neodymium concentration and (11) H. Samelson, A. Lempicki, and V. A. Brophy, J. Quantum Electron., 4, 849 (1968).
1789
STERICAND CONJUQATIVE EFFECTS IN PHENYL-SUBSTITUTED CATIONS output reflectivities chosen were nearly optimum for energy output for the SeOC12-based system whereas it has not been established whether they are also optimum for POCls. Under these conditions thresholds occur in the region of 200 J (SeOClz some 10% lower, Poc13 107, higher) ; however, with narrow-bore cells and higl er reflectance output mirrors, both SeOClz- and POCla-based systems have shown thresholds as low as 10-J input.
Conclusions On the basis of these experiments, it is clear that the
differences between the two liquid laser systems are not large. The POCh system starts with the advantages of lower corrosiveness and toxicity and under one set of experimental conditions shows a higher slope efficiency of power output. Nevertheless, its over-all decay time of fluorescence is lower (some 20% below what is calculated from spectroscopic considerations), and therefore it presumably has a lower quantum efficiency than has the SeOClz system. On the whole, however, both systems are quite similar in their physical characteristics and the choice of one or the other would be dictated largely by experimental conditions.
The Balance of Steric and Conjugative Effects in Phenyl-Substituted Cations, Radicals, and Anions by Roald Hoffmann, Robert Bissell, and Donald G. Farnum Department of Chemistry, Cornell University, Ithacu, New York 14860 (Received December 9, 1988)
The delicate balance of steric and conjugative effects which determines the equilibrium geometry of phenylsubstituted carbonium, allyl, pentadienyl, cyclopropenyl, cyclopentadienyl, and cycloheptatrienyl cations, radicals, and anions is studied by means of extended Hiickel calculations. Despite some doubts as to the applicability of this method to such species, the resulting potential energy curves are reasonable. In triphenylcarbonium ion there is found no evidence for a local minimum other than that in the propeller geometry. In diphenylcarbonium ion the transition from the equilibriumgeometry to its enantiomeris most easily accomplished through a transition state in which one phenyl group is coplanar, the other perpendicular. I n the triphenylcarbonium ion the corresponding conversion is estimated to proceed through a transition state in which all phenyl rings are perpendicular. In all cases the anions are predicted to be conformationally more stable than the cations. Hyperconjugative charge transfer occurs to ortho carbons of a phenyl ring as it is twisted out of conjugation. The potential energy curve for twisting the phenylcyclopropenium cation out of planarity is calculated to be very flat. This insensitivity of energy to rotation is not matched by absence of charge transfer. Several theoretical explanations for the calculated insensitivity of phenylcyclopropenium energy to twist of phenyl group are discussed, but none is completely satisfactory.
Introduction Phenyl groups may stabilize carbonium ions, radicals, and carbanions. This stabilization is a consequence of delocalization and is achieved through a .rr-type interaction, The maximum effect will be obtained when the phenyl group is coplanar with the adjacent ion or radical center, assumed trigonal. When several phenyl groups are attached, an all-planar geometry often is made impossible by short H-H contacts. The equilibrium conformations of these molecules are then determined by a delicate balance of steric and conjugative effects. It is this balance that we explore in this paper for
(PhC3H2+), diphenyl- (PhzCaH+) , and triphenyl(Ph3Cs') CYclOProPenium cations, PhenYlcYcloPentadienide anion (PhC5H4-), PhenYltroPYlium ion (PhC7 HB'), and Phenylallyl (PhCaH4) and PhenYlPentadienyl (PhC6Hd species.
Ary]carbonium Ions, Radicals, and Carbanions That planar conformations of P b C H and Ph3C are sterically impossible was apparent to some early workers in the field. The first clear suggestion of rotated geometries seems to be due to Lewis and coworkers1 and to SeeL2
volume 73, Number 6
June 1960
