Measurements of second- and third-order nonlinear polarizabilities for HF and HCl

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Abstract
Measurements of second- and third-order nonlinear polarizabilities (hyperpolarizabilities) for HF and HCl using dc electric-field-induced second-harmonic generation are presented: χ(3)∥(HF)=70(10)×10−39 esu/mol, χ(2)∥ (HF)=−4.70(41)×10−32 esu/mol, χ(3)∥(HCl)= 347(15)×10−39 esu/mol, χ(2)∥(HCl)= −4.22(50)×10−32 esu/mol. In the case of HF this allows a critical comparison with theory. HF has fewer electrons than any polar molecule previously studied experimentally and the small size of HF has made it an attractive candidate for theoretical investigation. Christiansen and McCullough have used numerical Hartree–Fock techniques to establish generally accepted criteria for basis set selection; and Bartlett and Purvis have applied to HF the most elaborate technique applied so far to the calculation of any molecular hyperpolarizability (CHF SDQ-MBPT[4]). Experimental corrections and uncertainties are carefully considered as are several other factors relevant to a comparison of these experimental and theoretical data. The theoretical results are about a factor of 2 smaller than the experimental data and none of the factors considered seems to offer a resolution of this discrepancy.