Alexander D. Trifunac

Radiation and photochemistry of onium salt acid generators in chemically amplified resists

The difference in photochemistry and radiation chemistry of sulfonium salt acid generator was investigated by product analysis and time resolved spectroscopic methods for chemically amplified resist application. After KrF excimer laser and electron beam irradiation of sulfonium salt, yields of decomposed products including acid were determined. The ultra fast in-cage reactions after laser irradiation were directly observed by the femtosecond laser flash photolysis method. Intermediates after electron beam irradiation were observed by nanosecond electron beam pulse radiolysis. From both the product analysis and time resolved spectroscopic methods, the contribution of each reaction pathway to acid generation was determined.

Nanosecond time‐resolved EPR in pulse radiolysis via the spin echo method

The design and operation of a time‐resolved electron spin echo spectrometer suitable for detecting transient radicals produced by 3 MeV electron radiolysis is described. Two modes of operation are available: Field swept mode which generates a normal EPR spectrum and kinetic mode in which the time dependence of a single EPR line is monitored. Techniques which may be used to minimize the effects of nonideal microwave pulses and overlapping sample tube signals are described. The principal advantages of the spin echo method over other time‐resolved EPR methods are: (1) Improved time resolution (presently ∼30–50 nsec) allows monitoring of fast changes in EPR signals of transient radicals, (2) Lower susceptibility to interference between the EPR signal and the electron beam pulse at short times, and (3) Lack of dependence of transient signals on microwave field amplitude or static field inhomogeneity at short times. The performance of the instrument is illustrated using CIDEP from acetate radical formed in puls...

Electron T1 measurements in short‐lived free radicals by dynamic polarization recovery

A dynamic polarization recovery method for measurement of electron spin T1 relaxation times in free radicals in liquids is described, which is valid even in the presence of chemically induced dynamic electron polarization (CIDEP) and fast chemical decay of the radicals. The method is based on pulsed microwave perturbation and detection of transient magnetization following radical creation in a short pulse. Analysis of the experimental approach and a theoretical description of the method is presented together with a detailed discussion of the advantages and the limitations of the technique. Electron T1 measurements are presented for 14 short‐lived free radicals generated in aqueous solution. The magnitudes of the observed relaxation times, which range from 0.1 to 4 μs, are discussed within the framework of current theories of relaxation for small radicals in liquids. It is tentatively concluded that the spin rotation mechanism is responsible for the very short T1’s in this series of radicals.

Optically detected time resolved epr of radical ion pairs in pulse radiolysis of liquids

Abstract Time resolved EPR spectra of aromatic radical ion pairs are measured by optical detection of microwave modulation (ODMR) of the fluorescence of excited aromatics from recombination of radical ion paris produced by pulse radiolysis ODMR data in solutions of pyrene in decalin indicate that geminate recombination processes are the major source of the excited states.

Pulse radiolysis of cyclohexane: The identity of the high-mobility positive ion

Abstract Previous experimental observations and new pulse radiolysis results concerning the high-mobility positive ion in cyclohexane are examined. Apparent disagreement among these observations concerning the existence of a high-mobility positive ion can be explained if there are two positive ions of major importance in cyclohexane radiolysis. The possibility is examined that the c -C 6 H 12 + ion has a “normal” mobility and that hydride transfer or proton transfer, involving c -C 6 H 11 + or c -C 6 H 13 + respectively, is responsible for the observed high mobility.

Magnetic resonance studies on radiation-induced point defects in mixed oxide glasses. I. Spin centers in B2O3 and alkali borate glasses

Radiation-induced spin centers in vitreous boron trioxide and alkali borate glasses were studied using pulsed electron paramagnetic resonance (EPR). It is shown that electrons and holes in these glasses are trapped on valence alternation defects, undercoordinated oxygen (holes) and overcoordinated oxygen (electrons). The local environment around these defects has major effect on spin parameters of the corresponding spin centers. The electronic and atomic structure of spin-1/2 centers and their diamagnetic precursors is analyzed using semiempirical and ab initio calculations.

Spin polarization mechanisms in early stages of photoinduced charge separation in surface-modified TiO2 nanoparticles

Abstract Steady-state and time-resolved electron paramagnetic resonance (EPR) experiments reveal that photoinduced charge separation in surface-modified nanoparticles yields interacting electron–hole radical pairs with spectral features indicative of a range of dynamic properties (Radical Pair Model and emissive Triplet Model CIDEP). Fast exchange in the radical pair is indicated by the presence of a central line at the midpoint, (gave=(gh+ge)/2) , of electron and hole g-factors. Excess emission is consistent with the triplet character of the exciton precursor. A subset of electron–hole radical pairs exhibits the spin feature characteristic of correlated radical pair polarization (CRPP) reflecting a weak interaction between photogenerated holes and electrons.

Submicrosecond time-resolved EPR in laser photolysis

Abstract Direct detection time-resolved EPR experiments were carried out in liquids by laser photolysis. Time resolution of 50–100 ns was achieved using a conventional EPS spectrometer. By observing spectra exhibiting CIDEP at various time windows after the laser pulse one can readily ascertain which CIDEP polarization mechanism is important. The spectra of the monoprotonated durosemiquinone radical, monoprotonated p -benzosemiquinone radical and the radicals formed during photolysis of benzophenone in isopropanol were examined.

Nanosecond time resolved epr spectroscopy. EPR time profile via electron spin echo. CIDEP

Abstract Spin-echo method has been applied to the study of time profiles and CIDEP in transient radicals produced by laser photolysis. Resolution was ≈ 20–30 ns and observations made even during the laser pulse. Benzoquinone, duroquinone and benzophenone in isopropanol were examined; EPR spectra at short times and kinetic traces of prominent peaks are illustrated.

Submicrosecond studies in pulse radiolysis by time-resolved EPR spectroscopy

Abstract Instrument improvements allow the study of transient radicals in pulse radiolysis of aqueous acid and alcohols in the 200–300 ns region after the electron pulse. The radical pair mechanism of CIDEP is clearly dominant in these short times and it appears that the residual spin population from the radical precursors can be observed.