EFFECT OF DENSITY ON THE INDUCED NEAR INFRARED SPECTRUM OF SOLID PARAHYDROGEN WITH ORTHO IMPURITIES AT 4.2 K

(1)

HAL Id: jpa-00224340

https://hal.archives-ouvertes.fr/jpa-00224340

Submitted on 1 Jan 1984

HAL

is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire

HAL, est

destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

EFFECT OF DENSITY ON THE INDUCED NEAR INFRARED SPECTRUM OF SOLID

PARAHYDROGEN WITH ORTHO IMPURITIES AT 4.2 K

A. Coulon, M. Jean-Louis

To cite this version:

A. Coulon, M. Jean-Louis. EFFECT OF DENSITY ON THE INDUCED NEAR INFRARED SPEC-

TRUM OF SOLID PARAHYDROGEN WITH ORTHO IMPURITIES AT 4.2 K. Journal de Physique

Colloques, 1984, 45 (C8), pp.C8-211-C8-212. �10.1051/jphyscol:1984839�. �jpa-00224340�

(2)

JOURNAL DE PHYSIQUE

Colloque C8, supplément au n ° l l , Tome *5, novembre 1984 page C8-2I1

EFFECT OF DENSITY ON THE INDUCED NEAR INFRARED SPECTRUM OF SOLID PARAHYDROGEN WITH ORTHO IMPURITIES AT 4.2 K

A. Coulon and M. Jean-Louis

Labovatoive des Interactions Moleculaires et des Routes Pressions, C.N.R.S, avenue J.B. Clement, 93430 Villetaneuse, France

Résumé - L'effet de densité sur le spectre proche infrarouge du parahydrogène dopé en impuretés ortho à 4,2 K et jusqu'à 10 GPa, se traduit par un élargis- sement et un déplacement des raies vers les hautes fréquences. Une forte aug- mentation de l'intensité d'absorption et la disparition de la raie S,(0)

sont également observées.

Abstract - The increase of the density on the near-infrared absorption of parahydrogen with ortho impurities at 4.2 K and up to 10 GPa, leads to a broadening and a blue frequency shift of the lines. A large enhancement of the intensity absorption and the disappearance of S1(0) line are also observed.

In the study of intermolecular interactions between hydrogen molecules, the effect of the density on the infrared absorption spectrum of solid hydrogen presents some interest. In the solid, the intermolecular forces give rise to an induced rotation- vibration spectrum /1/ which is very sensitive to the variation of intermolecular distance / 2 / . Particularly, the study of the effect of the density on the infrared spectrum of parahydrogen doped in ortho impurities, can bring some informations on the intermolecular potential and its vibrational dependence. The crystalline structure remains h.c.p. under pressure and the equation of state of solid p.H2 is known /3,4/.

This investigation has been carried out with a previously described apparatus using two opposed sapphire anvils cooled at liquid helium temperature /5/. The spectra of the figure 1 show the evolution with the pressure of solid parahydrogen for an ortho concentration of about &%. The spectra were recorded with a slitwidth of 5 c m " . With increasing density, all the lines are shifted toward higher frequencies with nearly the same features. A large enhancement of their intensity absorption and a broadening are observed. The magnitude of those effects depends on the nature of the transition.

The small band, labelled (A) on the figure 1, exhibits in fact several components unresolved with the experimental slitwidth. The origin of this band arises from several causes. Pure vibrational transition Q-] (0) associated with parahydrogen molecules leads to the vibron band. Owing to the rotation-vibration interaction, trapped ortho molecules monomers give rise to a Q-) (1) line shifted with respect to Ql (0) / 1 / . Beside

those lines, orthodimers leads to a set of lines, shifted towards the high frequency side with respect to the Qi(1) line. This shift depends on the intermolecular distance /6/. Moreover due to the concentration in orthomolecules (58!), Xmers (X>2) give rise to a supplementary absorption whose the shift is intermolecular distance dependent.

Therefore it results that the pressure observed broadening is chiefly due to the concentration used here. The maximum of the absorption associated with the band labelled (A) can be referred to the Q1 (0) line which is the most intense line of the absorption spectrum. This line is shifted toward higher frequencies with a pressure dependence similar to the previously observed shift in normal hydrogen /5/. The broad band Q R ( 0 ) (labelled (D) on the figure 1) arises from transitions for which the vibrations of molecules are coupled with lattice vibrations. As for normal hydrogen /5/ the frequency shift is important.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984839

(3)

JOURNAL DE PHYSIQUE

22 kbor

4100 4300 4500 4700 4900

WAVE NUMBER

Fig. 1. Effect of pressure on the near-infrared absorption spectrum of solid parahydrogen with ortho impurities (-8%) at 4.2K.

The pressure broadened band labelled (C) on the figure 1 corresponds to the simulta- neous transitions Ql(0) + So(0). In the region where there is no overlap with the

@(0) , line, its bandwidth varies as R-5 assuming a quadrupolar dependent width /6,7/.

In splte of the lack of accuracy due to the broadening, this line appears to be shifted with the same magnitude as Ql(0). The rotation-vibration line Sl(0) (labelled as

(B) on the figure 1.) is a sharp line which desappears fully in the broadened Q1(0) + So (0) band at about 65 kilobars. S1(0) line corresponds to a transition for which rotational and vibrational excitations are localized on the same molecule.

Due to the rotation-vibration interaction, the energy levels appear to be sharp levels under the _Ql(0) + So(0) energy band /6,7/. The rotation-vibration interaction does not seem to undergo large pressure change, therefore the levels disappear in the broadened energy band.

References

-

I/ H.P. GUSH, W.P.J. HARE, E.J. ALLIN, H.L. WELSH, Can. J. Phys. 38 (1960) 176.

2/ M. JEAN-LOUIS, J. Low Temp. Physics, 45 (1 981) 271.

3/ I.F. SILVERA, R.J. W I J N W E N , Phys. Rev. Letters, 47 (1981) 39.

4/ J. van STRATEN, R.J. W I J N O E N , I.F. SILVERA, Phys. Rev. Letters, 48 (1982) 97.

5/ A. COULON, M. JEAN-LOUIS, Proceedings of the VIIth International _AIRAFTConference, Le Creusot 1979, p. 598.

6/ J. van XRANENDONK, G. KARL, Rev. Mod. Phys. 40 (1968) 531.

7/ J. van U W E W Q N K , Physica 25 (1 959) 1080.