Bret C. Hess

Electroabsorption in C60 and C70. Third-order nonlinearity in molecular and solid states

Abstract We have studied electroabsorption (EA) in both films and disperssed molecules of C 60 and C 70 . In contrast to a recent model for the third harmonic generation (THG) spectrum, we find that the two-photon states important to the nonresonant χ (3) (0) in molecular C 60 are above the 3.6 eV T 1u state, not below it. We present a model consistent with spectra of EA, THG and degenerate four-wave mixing (DFWM). The lowest-energy resonances in χ (3) ( ω ) are dominated by ‘forbidden’ transitions made allowed by vibronic coupling in the molecule and intermolecular coupling in the solid.

Optically detected magnetic resonance (ODMR) of polarons and charge-conjugation symmetry in conducting polymers

Abstract The main narrow photoluminescence(PL)-enhancing g = 2.0025 ± 0.0010 X-band ODMR of poly(3-alkylthiophene) (P3AT) and poly(2,5-dialkoxy para phenylenevinylene) (PDOPV) films, solutions, and blends is described and discussed. The resonances all closely match a sum of narrow and broad Gaussians. In P3AT the narrow Gaussian saturates at a lower power than the broad Gaussian. While this ODMR is believed to result from intrachain ‘distant pair’ polaron recombination, the assignment of each Gaussian to the proper polaron is undecided. The g -values of the two Gaussians are nearly identical in PDOPV, apparently reflecting a very high degree of charge-conjugation symmetry (CCS). They are, however, quite distinct ( g = 2.0032, 2.0019, respectively) in P3AT, possibly due to the sulphur heteroatom on the thiophene rings. The ramifications of this CCS violation for the PL quantum yield are discussed. In all cases, the spectral dependence of the narrow resonance is similar to the PL spectrum, indicating that the polaron levels are close to the band edges and to the levels of the (presumably) short-lived geminate singlet excitons. The sensitivity of the ODMR intensity to sample processing suggests that it may possibly be affected by an unidentified native, possibly conformational, defect. The decay modes competing with polaron recombination are also discussed.

Effect of pressure on photoluminescence and optical absorption in meh-ppv

Abstract We have studied the photoluminescence (PL) and optical absorption in MEH-PPV as a function of pressure up to 50 kbar. The PL and optical absorption bands redshift with pressure and the PL efficiency decreases dramatically. These features are similar to results reported earlier for pressure effects in unsubstituted PPV. However the initial pressure redshift in MEH-PPV is about twice as large as that in unsubstituted PPV, and the PL decays faster. The vibronic structure in the PL is maintained up to the highest pressures,

Optical properties of polydiethynylsilanes: A quasi (AB)x polymer

Abstract We have measured optical absorption, resonant Raman scattering (RRS), cw photomodulation (PM), picosecond transient PM and degenerate four-wave mixing, in I 2 -doped and undoped polydiethynylsilanes (PDES): [(HC = C) 2 SiR 2 ] x , where R = alkyl, phenyl, etc. Our measurements show that the PDES backbone structure consists of units of four-membered rings alternatively connected to CH and SiR 2 units and thus it can be regarded as an almost degenerate ground state (AB-AB) x conducting polymer. As such, it supports A and B solitons as primary excitations; these have been observed in doping and photoexcitation spectra. The non-linear x (3) value at 625 nm was measured to be 3 × 10 −9 esu, one of the largest measured in conjugated polymers.

Photoluminescence and optically detected magnetic resonance in polythiophene and poly(3-alkylthiophenes); substitution, disorder and ageing effects

Abstract The influence of side-group substitution, disorder, and ageing in the photoluminescence (PL) and related optically detected magnetic resonance (ODMR) has been studied. Alkyl substitution causes energy shifts and broadening in the PL spectra. Ageing and disorder has a profound effect on the PL and PL excitation spectra, and a possible effect on the temperature dependence of the ODMR intensity. Our results suggest that ageing substantially increases the subgap absorption tail and this in turn displaces the PL spectrum to lower energies due to reabsorption.

Studies of photocarrier relaxation processes in polysilane alloys from subpicosecond to metastability

The photocarrier relaxation processes in polysilane alloys, a-Si:H/a-(SiH2)n, have been studied using the picosecond transient and cw photomodulation (PM) techniques and light-induced ESR. For below gap excitation, the picosecond decay is exponential with an increasing decay rate at lower temperatures. This is interpreted in terms of e-h geminate recombination in the small clusters (≈ 10 A) of a-Si:H embedded in the polysilane matrix. The cw PM spectrum contains a single photoinduced-absorption band, which is interpreted as due to photocarriers trapped in dangling bond (DB) defects. This interpretation is based on the temperature and excitation intensity dependencies, as well as on the reduction of the ESR signal under illumination. From the analysis of the DB optical transitions, the DB effective correlation energy Ueff = 0.3 eV and its relaxation energy Δ Er = 0.4 eV are estimated.

Photoexcitation of spin states in conducting polymers studied by the spin-dependent photomodulation technique

Abstract The nature of various photoinduced absorption (PA) bands in conducting polymers is still not completely resolved. We have introduced a novel technique, spin-dependent photomodulation (SDPM), in which we measure changes in c.w. PA due to resonant absorption of microwaves in magnetic fields, to elucidate the spin states associated with the photoexcitations related with the PA bands. We have used the SDPM technique to study spin states in soluble trans-(CH)x and oriented poly(paraphenylene vinylene) (PPV) films. We have verified that the long-lived charged excitations are associated with spinless features in PA whose recombination (or generation) is spin dependent. These are charged solitons in (CH)x and charged bipolarons in PPV. The stable neutral excitations, on the other hand, are self-trapped triplet excitons in the non-degenerate ground state PPV and neutral solitons in the degenerate ground state (CH)x.

Nonresonant magnetic field effects in the photoluminescence of polythiophene and poly(3-alkylthiophenes)

Magnetic fields (10 < B < 700 G) applied to polythiophene and its derivatives cause changes in the photoluminescence (PL) intensity. In chemically coupled polythiophene, sharp peaks occur at 110 and 220 G. These peaks may be due to the coupling of photogenerated spins to nuclear quadrupole transitions. This in turn suggests that recombination sites at the end of the chains are important in the PL process. Much broader features in the PL vs. B spectrum are also mentioned and discussed.

Picosecond and c.w. optical probes of polydiacetylene at high pressure

Abstract We have employed the femtosecond photoinduced absorption (PA) technique at pressures up to 80 kbar in films of polydiacetylene 4BCMU. Pressure dramatically slows the singlet exciton recovery, suggesting that large-amplitude 3-D distortions of the chain are important to the picosecond recovery. At high pressure the slow PA decay can be modeled by diffusion-limited recombination of excitions. On the other hand, the initial 100 fs relaxation is not affected by pressure, and is identified as a 1-D process. We also present pressure studies of resonant Raman scattering (RRS) in 4BCMU. Amplitude-mode analysis of the RRS data shows that the 1-D properties of the excitons change very little with pressure. Changes in the RRS include: (1) hardening of the bare phonons; (2) softening of the distribution of the dressed phonons in parallel with the red shift of the absorption bands under pressure. These observations are consistent with an increase in the average conjugation length with pressure.

High pressure effects in picosecond & C.W. spectroscopies of conjugated polymers

Abstract We have studied polydiacetylene(PDA) and polythiophene films under hydrostatic pressure up to 60 kbar. In PDA-4BCMU, the excitonic recovery time increases from a 1.5 psec exponential decay at atmospheric pressure, to a nonexponential decay over several decades in time at high pressures. In poly (3-hexylthiophene), (P3HT), the absorption and photoluminescence (PL) bands exhibit similar redshifts that saturate near 40 kbar and the thermochromic effect is completely inhibited at high pressures. We suggest that these effects in both PDA and P3HT polymers are due to the inhibition of out-of-plane rotations by the application of pressure.