Freejet fluorescence excitation spectrum of t r a n s, t r a n s1,3,5,7octatetraene J. Chem. Phys. 75, 4338 (1981); 10.1063/1.442640Experimental confirmation of the dipole forbidden character of the lowest excited singlet state in 1,3,5,7 octatetraene J.The IIBu~1 lAg fluorescence spectrum of trans, trans 1,3,5,7-octatetraene in a supersonic helium jet has been determined. Excitation at the 1 IBu origin (35 549.3 cm -I) gives rise to a well resolved emission spectrum whose vibronic development is dominated by the single and double bond stretch modes of the polyene. There is no gap between the onset of the dispersed emission spectrum and origin of the previously reported fluorescence excitation spectrum. Observed band profiles are well described by the convolution of 12 cm -I Lorentzians with the spectrometer response function. The fluorescence lifetime for excitation at the liB u origin is shorter than 2 ns; an excited state decay time of 100--300 ps follows from an analysis ofthe dependence of fluorescence signal on excitation power in the nonlinear regime. The rough mirror symmetry between the 0-0 excited emission and excitation spectra and the short lifetime indicate that the emission comes exclusively from the 1 IBu state with no significant contribution from the lower lying 2 lAg state. For excitation at the origin the overall emission yield is estimated to be 0.05-O.IS.

Highly resolved emission and one-photon fluorescence excitation spectra for 1,4-diphenyl-1,3-butadiene seeded in a supersonic expansion of helium have been measured. The spectra show a long-lived (52.8 nsec for excitation at the 0-0) state at 29,652.5 cm-, approximately 1,150 cm-' below the well-characterized 'Bu state, which is assigned as 'Ag-i.e., we have directly observed a polyene 1Ag state in the gas phase. Emission spectra and decay times for the 1Ag state were measured at a number of different excitation energies. These data clarify the ordering of excited singlet states and the photophysical behavior of diphenylbutadiene.The connection between electronic structure and photochemical behavior in the linear polyenes is the focus of a number of spectroscopic and theoretical investigations (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Particularly for the shorter polyenes, the photophysics of these molecules is dramatically affected by the close proximity of the two lowexcited singlet states (21Ag and 11BJ). In fact there is evidence that the separation and even the ordering of these states is a strong function of effective conjugation length and molecular environment (8,12,(15)(16)(17). While high-resolution spectroscopy in condensed and gas phases has greatly increased our insight into the photochemical behavior of polyenes, in particular octatetraene (8, 9), less is known about the low-lying singlet states of hexatriene and butadiene. This is primarily because of the lack of detectable fluorescence (8), which is not a problem for the a,w-diphenylpolyenes.Despite the fact that 1,4-diphenyl-1,3-butadiene fluoresces with reasonable yield, there are still some outstanding questions regarding the proper description of the low-lying singlet states of this molecule. On the basis of one-and two-photon spectra measured for diphenylbutadiene in EPA at 77 K, the 21Ag state has been located at 27,900 cm-1, 130 cm'-below the origin of the l1B1h state (1). However, the two-photon fluorescence-excitation spectrum of diphenylbutadiene in low-temperature hydrocarbon glasses and solutions can be interpreted as showing an 'Ag state higher in energy than the 1B, (11,14). This discrepancy is not explained by simple solvent-shift theories even though these theories quantitatively account for the solvent dependence of the ground state-to-l1Bu transition energy (12). The measured lifetime of the emitting state of diphenylbutadiene in cyclohexane at room temperature is 1.8 nsec, which is consistent with either state ordering (10). Molecular orbital calculations using configuration interaction through double excitations predict at least two low-lying 1Ag states; one at 28,000 cm'1, which has roughly 20% polyene character, and one at 30,440 cm-', which has roughly 45% polyene character (1). Thus, not only is the ordering of the 'Ag and 1Bu states at issue, but exactly which 1A state is lowest in energy is also open to question. Bennett and Birge (1) have assigned the state observed in their two-photon spectrum as a trans...

The lithographic performance of chemically amplified positive resists depends on controlling the delay time between the exposure and post‐exposure bake (PEB) process steps. The effect of post‐exposure delay (PED) was investigated in poly(t‐butoxycarbonyloxystyrene‐sulfone) formulated with photoacid generators (PAG) by a variety of analytical techniques including Rutherford backscattering, infrared, X‐ray photoelectron and laser ablation microprobe mass spectrometry. The results indicate that the base‐insoluble residue formed during PED is primarily a surface phenomenon and is a result of incomplete deprotection at the resist surface. It was determined that ppb levels of basic vapors present in the resist processing environment affect process performance profoundly by reacting with the photogenerated acids during PED. Isolation of resist surface from the immediate environment by means of a thin, acidic, and base‐soluble overcoat (covercoat) material dramatically alleviates the environmental effects by quenching basic airborne contaminants and improves PED latitude.

Separate fluorescence excitation spectra of the blue- and UV-emitting forms of methyl salicylate cooled in a free-jet expansion are reported. This study represents the first observation of the detailed vibrational structure of these transitions. The two excitation spectra have no features in common, and their intensity patterns are very different. Many individual lines are ∼2 cm−1 wide (nearly laser limited), although in the excitation spectrum of the UV emission, spectral congestion persists at high energies despite the high degree of cooling. (AIP)

The fluorescence excitation spectrum from 2980 to 2650 Å of trans, trans-1,3,5,7-octatetraene in a free jet has been measured monitoring emission in the region 3250 to 3500 Å. The vibronic bands of the 1 1Bu←1 1Ag (S2←S0) transition of octatetraene narrow significantly in a seeded free jet of ∼8 ppm octatetraene in helium (FWHM∼200 cm−1 for the room temperature static gas; FWHM∼35 cm−1 in the jet). Although absolute intensities of the vibronic bands are not determined, there is significant intensity in the origin, and the vibronic development involves only a few modes. Estimated translational, rotational, and vibrational temperatures of ∼1,<45, and <170 K, respectively, were achieved in the expansion. The major source of the observed linewidth is attributed to coupling of the 1 1Bu state to the lower 1Ag states.