J. L. Hunt

Raman scattering from polymerizing styrene. I. Vibrational mode analysis

The frequencies and intensities of the Raman spectrum of thermally polymerizing styrene were measured in order to confirm (or correct) the mode assignments in the literature and to observe the effect of polymerization on the Raman active modes. The spectra were measured over a period of three weeks while an originally highly purified sample of monostryrene was kept at about 100 °C. The polymer vibrations are identified with their monomer ’’origins’’ which are in turn compared to known benzene (or benzene derivatives) and ethylene (or vinyl group) vibrations.

The pressure variation of the glass transition temperature in atactic polystyrene

Brillouin frequency shifts from light scattering are used to measure the variation with pressure of the zero‐frequency glass transition temperature T g in atactic polystyrene (PS) at pressures up to 3 kbar. These results are extrapolated to high pressures and compared with theoretical predictions. Experimentally it is found that T g asymptotically approaches a critical value at high pressures. The equilibrium lattice model of DiMarzio, Gibbs, Fleming, and Sanchez which approximates the residual entropy of a polymeric glass correctly predicts the experimentally observed trends. The molecular theory of Bengtzelius, Gotze, and Sjolander is also qualitatively correct at high pressures even though it deals with the glass transition of a classical monatomic fluid. Neither theory is quantitatively in agreement with our results.

Raman scattering from polymerizing styrene. II. Intensity changes as a function of conversion

The Raman spectrum of thermally polymerizing styrene has been recorded for temperatures of 100 and 70 °C. The intensities of the ν9 (C = C stretch mode) and ν14 (C–H bending mode) in the vinyl group are used as an indicator of the percent conversion from monomer to polymer. These intensities, normalized to that of ν24 (phenyl breathing mode), are compared to the depolarized Rayleigh intensity of polymerizing styrene. It is suggested that there is very little orientational correlation between the polymer and monomer molecules.

Raman spectra at low temperatures and depolarization ratios for styrene and polystyrene

The Raman spectra of styrene and atactic polystyrene have been recorded at 85 and 225 K. As the temperature is lowered, all 42 vibrational modes of the styrene molecule were resolved including those normally of very low intensity in Raman scattering. A general increase in the intensity of all lines and in the number of overtones and combination bands was observed. The frequency of out‐of‐plane vibrational modes was increased due to increased steric hindrance, whereas the frequency of in‐plane modes remained relatively stable. Of particular interest are 11 low frequency modes, most of which are believed to track low frequency benzene‐vinyl torsional oscillations from gaseous styrene and are enhanced by the crystal field. Depolarization ratios were measured for styrene at 300 K and polystyrene at 300 and 85 K. We have confirmed that there is some correlation between very symmetric vibrational modes in styrene and atactic polystyrene and strong polarization (ρV<0.2), and between very asymmetric vibrational m...

Infrared spectra of liquid and solid HT and HD in mixtures with T2

The collision‐induced fundamental vibration–rotation spectra of liquid and solid HT and T2 in a mixture of 50% H and 50% T have been recorded. The spectra of liquid and solid HD in a 90% HD plus 10% T2 mixture have also been observed. The frequencies of the numerous single and double transitions have been compared with those calculated from the known molecular constants. Deep holes have been observed in the Q (O) phonon band of solid HD, HT, and DT. The position and extent of these holes have been related to a theoretical treatment which attributes the holes to a coupling of the rotational and translational motion of the molecules through the anisotropic part of the intermolecular interaction. The effect of J=1 impurity molecules on the intensity of the sharp Q1(O) line of liquid HD is discussed and the effect of the radioactive decay heat on the temperature of the liquid and solid samples is evaluated.The collision‐induced fundamental vibration–rotation spectra of liquid and solid HT and T2 in a mixture of 50% H and 50% T have been recorded. The spectra of liquid and solid HD in a 90% HD plus 10% T2 mixture have also been observed. The frequencies of the numerous single and double transitions have been compared with those calculated from the known molecular constants. Deep holes have been observed in the Q (O) phonon band of solid HD, HT, and DT. The position and extent of these holes have been related to a theoretical treatment which attributes the holes to a coupling of the rotational and translational motion of the molecules through the anisotropic part of the intermolecular interaction. The effect of J=1 impurity molecules on the intensity of the sharp Q1(O) line of liquid HD is discussed and the effect of the radioactive decay heat on the temperature of the liquid and solid samples is evaluated.

Helium emission spectra from doped samples of solid hydrogen and deuterium

Using proton‐beam irradiation at 15 MeV, the emission spectra from 3He and 4He doped samples of solid hydrogen and deuterium have been observed from 350–750 nm at 4.2 K. Such samples simulate the conditions of solid tritium or tritiated hydrogens after about six days of helium accretion. The spectra are very similar to pure helium gas spectra acquired at 4.2 K and closer still to spectra acquired using helium gas in the upper portion of a sample cell partially filled with solid hydrogen. The helium collects as macroscopically large bubbles whose pressure has been determined. The pure 4He gas spectrum at 4.2 K contains a band near 460 nm previously assigned to an unspecified excimer–dimer radiative transition. The pressure and isotope dependence of this band is presented which lends support to that interpretation. A mechanism is proposed for the transition.

Helium hydride emission spectra at 550 and 640 nm

Emission spectra of three isotopomers of helium hydride (4HeH, 3HeH, and 3HeD) in the visible spectral region have been acquired using proton‐beam irradiation of dense helium gas (150 Torr) at 4.2 K in the presence of some solid hydrogen or deuterium. Besides the previously reported D 2Σ+→A 2Σ+ transition, near 550 nm, a second transition near 640 nm, identified as the D 2Σ+→B 2Π, has been acquired and analyzed. The spectroscopic constants for both transitions have been obtained and compared to the theoretical results based on the latest published potential curves. Further insight into the mechanism for forming HeH will be presented, which indicates that the formation process is sensitive to the hydrogen vapor pressure above the solid.

Collision induced infrared lines in solid hydrogens caused by tritium radioactivity

Abstract We report the first observation of infrared absorption lines due to the presence of radioactive tritium in crystals of the solid hydrogens. Two prominent lines appear at the low-frequency side of the collision-induced spectrum, and are interpreted as due to the presence of positive and negative ions or electrons created during the radioactive process.

Absorption spectra from high vibrational levels of He2

Absorption spectra of dense helium gas at cryogenic temperatures has been acquired while the sample was irradiated using a 6.5 MeV proton beam. By chopping the proton beam, rather than the source lamp, we were able to achieve one part in 104 spectral sensitivity. The spectra showed six new bands in 4He2 and three in 3He2. These have been identified as transitions between high‐lying vibrational levels, with the strongest originating on the highest bound level of the a 3Σ+u potential. The temperature and pressure dependence of these features, as compared to low‐lying molecular and atomic features, offers some insight into the reaction dynamics of this fundamental system.

Infrared emission spectra from cryogenic proton‐irradiated helium gas

Spectra from proton‐beam irradiated helium gas, near 4.2 K, have been examined in the near infrared spectral region using a sensitive photomultiplier‐grating spectrometer combination (750–1000 nm) and a Fourier transform infrared spectrometer with an InGaAs detector (800–1800 nm). Most of the observed features can be assigned to known He2 transitions, and two bands, not previously observed (d 3Σ+u(v=4)→c 3Σ+g(v=3,4)) have been identified and analyzed. Some of the unidentified spectra have qualitative similarity to a set of visible emission lines tentatively assigned to an excimer–dimer (i.e., He4, two He2 excimers weakly bound together). These features, unlike the normal He2 spectra, demonstrate pronounced dependence on both the pressure and temperature of the sample. A comparison of the intensities of the singlet and triplet D(d)→C(c) transitions to the singlet and triplet C(c)→A(a) transitions is made.