Piezomodulation

The second approach is to use low temperature piezomodulation spectroscopy to obtain a more detailed picture of the electronic spectra. In this case, the low temperature spectrum of PTS, which is known to show excitonic interaction, is studied. 

Pertinent experimental details are given in the following section after which are presented the spectra of a phenylurethane series of PDA crystals. The nature of the coupling between the transitions is pursued using these results. The subsequent section presents the low temperature piezomodulation spectrum for PTS. The nature of the vibronic structure in this spectrum and its relation to the electronic structure is discussed. 

Samples were provided by R. R. Chance then of Allied Corporation. The samples were used without further processing. Both the normal incidence specular reflection (15) and piezomodulated reflection experiments are described elsewhere (16). The size and physical character of the first three members of the phenylurethane series of PDA crystals prevented their investigation with our low temperature apparatus. 

Crystals for the low temperature piezomodulation experiments were placed in a uniaxial stress mount which was itself located in a CTI helium refrigerator. Spectra were measured in 10 or 25 cm intervals using a Jobin-Yvon THR-1500 double pass monochromator. 

Another approach to the investigation of the intermolecular polariton coupling, as well as to the effects of strain, is piezomodulation spectroscopy. In this experiment a periodic uniaxial stress, within the elastic regime, is applied to the sample and the optical signal is detected synchronously with the stress. The measurement can yield information on the coupling, the strain dependence of the spectra, and more extensively resolved structure (12). Because of sample size and stability, PTS was studied by piezomodulation spectroscopy at low temperature. 

A previous report has demonstrated how the temperature dependence of the piezomodulation spectra has been able to produce a more detailed picture of the interaction of the side chains with the polymer spine (23). It was shown that different vibronic modes became active at different temperatures for a uniaxial stress along the b-axis of the crystal. 

There are two quite striking differences in the b-axis and a-axis piezomodulation spectra. First, the sense of the strain response is reversed, i.e. the transverse frequencies appear as minima in the a-axis spectrum rather than as maxima as seen for the b-axis spectrum. Second, the a-axis structure virtually vanishes at frequencies greater than 16,000 cm ... 

The reversal of the response of the piezomodulation is not surprising since it arises from the extreme anisotropy of the system. It would be expected that an extensive stress for the polymer spine would appear as a compression for a direction perpendicular to the spine. 

The two minima for the a-axis piezomodulation spectrum occur at 15,610 and 15,860 cm thereby exhibiting the characteristic 250 cm energy difference typical of the doubled bands seen in the low temperature phase of PTS. 

Figure 2. Piezomodulated specular reflection spectra of PTS at 17K with light polarized along the b and a (b) axes. The uniaxial stress was along the b-axis.

Within the resolution of the experiment, these are the same values of the peaks in the piezomodulation spectrum for the b-axis. Thus, the Davydov splitting is less than 10 cm Characteristic shoulders are seen approximately 50 cm to the blue of the a-axis minima. Their origin is unknown but suggestive of a lattice mode. 

The small upper limit of the Davydov splitting established by the low temperature piezomodulation spectra of PTS indicates that the interchain coupling of electronic transitions is negligible for that system. While the sidechains of the phenylurethane series may change this somewhat, it is unlikely that the interaction will exceed the weak coupling case. This is confirmed by the bandwidth studies of the reflection spectra where the coupling may approach the intermediate case. 

Modulation spectroscopy deals with the measurement and interpretation of changes in the optical spectra when modifying the measurement conditions. This can be achieved by applying an oscillatory perturbation such as an electric (EA) or magnetic field, heat pulse (thermomodulation) or stress (piezomodulation). Since electroluminescent diodes are operated by the application of an external electric field, EA is a particularly pertinent and powerful type of spectroscopy for the study of these devices. 

The piezomodulated spectrum of a single crystal of POA-TS was measured with the applied strain in the polymer chain direction (8). As discussed above the primary effect of such a uniaxial strain is to shift the reflection spectrum linearly to higher energy (28). Thus it might be expected that the piezomodulated spectrum should approximate a sixple derivative of the reflection spectnam. Figure 11 shews that this is the case. Similau results were found for PDA-TCDa (11). 

Fig. 11 Direct (solid line) and piezomodulated (dashed line) reflection spectra of PTS at 30o K. Bandpass is 30 cm" ...

---