The optical spectra of the lanthanides doped into single crystal LaF 3 have been interpreted in terms oftransitions within 4fN configurations. Energy matrices combining free-ion terms with a crystal field for an approximate model which assumes C 2v instead of the actual C 2 site symmetry were diagonalized. Excellent correlations were obtained between experimental transition energies and the computed level structures. We also report the results of previously unpublished experimental spectroscopic investigations ofNd 3 + and Sm 3 +:LaF 3 , as well as predicted energy levels for Pm3+:LaF 3 . The spectroscopic data for each ion were independently interpreted using an effective-operator model, then the model parameters were intercompared. Systematic trends have been identified, and a comprehensive energy level diagram is presented.

Pr-* + :LaF-j with the Model Energy Level Calculation in the Range 4200-4900 cm-1 (the 3 H 6 State) at ~ 4 K 20 2. Comparison of the Experimental Absorption Spectrum of Pr :LaF-i with the Model Energy Level Calculation and with Previous Proposed Interpretations (A. Assignments of Ref. 48, B. Assignments of Ref. 45) of the Energy Level Structure in the Range 16800-17400 cm" 1 (the A D 2 State) at ~ 4 K 23 3. Comparison of the Experimental Absorption Spectrum of Pr :LaF-j with the Model Energy Level Calculation and with a Previous Proposed Interpretation (A. Assignments of Ref. 45) of the Energy Level Structure in the Range 20800-22000 cm-1 (the 3 PQ, l l 6 and 3 Pj States) at ~ 4 K 24 4. Comparison of the Experimental Absorption Spectrum of Pr 3+ :LaF.j with the Model Energy Level Calculation in the Range 22600-22900'cm-1 (the 3 P 2 State) at ~ 4 K 25 5. Comparison of the Experimental Absorption Spectrum of Tm 3H :LaF 3 with the Model Energy Level Calculation in the Range 5600-6000 cm-1 (the 3 F^ State) at ~ 4 K 33 6. Comparison of the Experimental Absorption Spectrum of Tm 3+ :LaF 3 with the Model Energy Level Calculation in the Range 8200-8600 cm-1 (the 3 H 5 State) at ~ 4 K 34 7. Comparison of the Experimental Absorption Spectrum of Tm :LaF 3 with the Model Energy Level Calculation in the Range 12500-12900 cm" 1 (the 3 H. State) at ~ 4 K 35 iv LIST OF FIGURES No. Page 8. Comparison of the Experimental Absorption Spectrum of Tm :LaF 3 with the Model Energy Level Calculation in the Range 14300-14700 cm" 1 (the 3 F 3 State) at ~ K 36 9. Comparison of the Experimental Absorption Spectrum of Tm 3+ :LaF 3 with the Model Energy Level Calculation in the Range 15100-15300 cm-1 (the 3 F 2 State) at ~ 4 K 37 10. Comparison of the Experimental Absorption Spectrum of Tm 3+ :LaF 3 with the Model Energy Level Calculation in the Range 28000-28200 cm-1 (the 1 B 2 State) at ~ 4 K 38 11. Comparison of the Experimental Absorption Spectrum of Tm 3+ :LaF 3 with the Model Energy Level Calculation in the Range 34500-37000 cm-1 (the 1 I 6 , 3 P 6 , and 3 Pj States) at ~ 4 K.... 39 12. Comparison of the Experimental Absorption Spectrum of Tm 3+ :LaF 3 with the Model Energy Level Calculation in the Range 38200-38600 cm-i (the 3 P 2 State) at ~ 4 K 40 13. Comparison of the Experimentally Observed and Model Computed Crystal-field Levels for the 5 I ? State of Ho 3+ :LaF 3 : (a) From Ref. 62, Table III, (b) Computed levels from Appen. VIII 43 14. Absorption Spectrum of Ho :LaF 3 at ~ 4 K in the Range 1880-1930 nm 44 15. Variation of the Parameters F 2 , F^, F 6 , and Zeta (in cm-1) for both Ln :LaF 3 and Ln J :LaCl 3 as a Function of the Number of f-electrons (N) 51 LIST OF FIGURES No.

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Photoionization yield curves have been obtained for the major ions from CH 3 SH. Auxiliary studies with selectively deuterated species reveal that CD 3 S+ has a slightly lower threshold than CDzSH+ from CD 3 SH; but CHzSD+ has a lower threshold than CH 3 S+ from CH 3 SD. An analysis of alternative interpretations is presented. The isotopic studies also imply that CHzS+ is more stable than HCSH+, and HCS+ is more stable than CSH+. The value..:l H'f,O(HCS+) ~259.2±0.9 kcal/mol obtained in this study is in good agreement with a bracketed proton affinity for CS, but differs significantly from a recent ab initio calculation. This result confirms the view that HCS+ can be formed in interstellar clouds by exothermic reactions. The reaction CH 3 S+ /CHzSH+ ~HCS+ + Hz proceeds weakly at threshold, and much more strongly after overcoming an activation barrier of -0.9 eV. This ion yield curve is shown in Fig. 3, again with an enlarged threshold region. A very weak onset is observed at -1170 'A (-10.6 eV), slowly increasing to -1070 'A (-11.6 eV), where it abruptly grows and develops a cusp. To shorter wavelengths it then remains relative-Here aH, aD refer to the statistical degeneraCies of the two reactions, the G*'s are the sum of states in the respective activated complexes (or transition states), and E is the internal energy relative to the activation energy