J. H. Jaffe

Theory and Measurement of Pressure‐Induced Shifts of HCl Lines Due to Noble Gases

Pressure shifts of HCl absorption lines in both the 1–0 and 2–0 bands caused by five noble gases have been measured. An outstanding feature of these measurements is that the shifts are not the same for all lines of a band, but there is a strong dependence on the rotational quantum number J. However, there is evidence that the shifts tend to a constant limit for high J numbers. In order to interpret these results induction‐ and dispersion‐type van der Waals forces were considered and used in conjunction with existing line‐shape theories. In cases where perturbation theory can be applied, the limit shifts were accounted for fairly well, although no satisfactory explanation has emerged for the J dependence. Attention is drawn to the significance of observations of this sort as a means of obtaining information about intermolecular forces and molecular constants. Of particular interest is the possibility of obtaining such information for molecules in specific quantum states.

Pressure Induced Shifts of HCl Absorption Lines in the Infrared

The shift of infrared absorption lines due to pressure changes are small and they are difficult to measure. However it has been found possible to study this effect with the large spectrometer in Rehovoth which is particularly suited for the determination of small wavelength differences. Pressure induced shifts of the individual lines of the HCl 2–0 band at 1.76 μ have been measured over a range of pressures below atmospheric. A strong dependence of the shift on the rotational quantum numbers was found. For self‐shifts the lines near the band center were shifted toward the blue and others toward the red. The results are discussed qualitatively. Measurements on shifts due to the addition of a foreign gas are also reported.

Theory of Pressure‐Induced Shifts of Infrared Lines

A treatment is presented that accounts to a large extent for the j dependence of pressure‐induced shifts due to noble gases of lines in HCl bands. After a brief critical review of the theoretical work done on this subject to date, an improved phase‐shift approximation for the intermolecular collision process is worked out with a coordinate system fixed in space during the collision. The main features of the observed phenomena are accounted for using only parameters calculated directly from known molecular properties. A discussion of the limitations of this theory is given. It appears that in order to achieve further improvement, the finite probability of j transitions and the influence of short‐range forces should be taken into account.

New Method of Measuring Linear Dichroism in the Ultraviolet: Application to Helical Polymers

A new method of measuring linear dichroism is described wherein the determination is carried out with a single spectral run without moving either sample or polarizer. It makes possible, for the first time, reliable quantitative measurements of dichroism. As examples, spectra (down to 180 mμ) of dichroic films of poly‐L‐lysine and of deoxyribonucleic acid are presented.

Pressure‐Induced Shifts of Hydrogen Fluoride Lines Due to Noble Gases

The pressure‐induced shifts of hydrogen fluoride 1–0 and 2–0 band lines due to noble gases were measured. The shifts have the same general features as those observed with HCl, but they display certain peculiarities that have been interpreted by using a phase‐shift theory with a particular collision model.

Infrared Dispersion of Liquids by Critical Angle Refractometry

A critical angle refractometer for liquids in the infrared is described. Arsenic trisulfide prisms are used. In determining the critical angle of refraction, the influence of fringes formed in the thin sample layer is taken into account. Dispersion data are presented for six solvents in the 1–6 μ region. A summary is given of the sources of published data on the infrared dispersion of solvents.

Pressure‐Induced Shifts of DCl Lines Due to HCl: Shift Oscillation

Pressure‐induced shifts of DCl lines (2–0 band) due to HCl have been observed. In addition to the expected J dependence there is a striking oscillation from line to line of the magnitude of the shift. It is suggested that this phenomenon is due to near‐resonant dipole interactions between HCl and DCl.

Infrared Dispersion of Absorbing Liquids by Critical Angle Refractometry

A critical angle method is described for determining refractive indexes of moderately absorbing liquids (k < 0.03). It is pointed out that when the sample is absorbing the value of the critical angle depends upon the extinction coefficient as well as the refractive index. Dispersion data are given for weak infrared bands of CHCl3 and CS2.

Concerning the Use of the Fabry-Perot Interferometer for Wave-Number Measurement in the Infrared

Many workers have used channeled spectra produced by a Fabry-Perot interferometer to calibrate infrared spectrometers. In this paper the effects of the finite height of the spectrometer slit and the finite aperture of the condensing lens on the visibility of the spectra are discussed. The way in which maximum accuracy of calibration is to be obtained for any given experimental arrangement is indicated.

The Measurement of Refractive Indexes of Liquids in the Infrared

A method of measuring the refractive indexes of liquids in the infrared is described. A Perkin-Elmer spectrometer is adapted for this purpose without interfering with its normal operation as a routine instrument. Although use is made of a Fabry-Perot etalon, the method is not interferometric in the accepted sense. The accuracy that has been obtained is 0.002 at a wavelength of 1μ, but in principle the possible accuracy of the method may be equal to that of the best goniometer.