OCR Text |
Show the red of 276 nm where the one-color REMPI spectra ends. In particular the origin of the S I-S 0 absorption, the transition from v"=O to the vibrationless level of the excited state (v'=O), should appear at -279.6 nm. The reason that this and other red bands are absent from the one-color REMPI spectra in the top two panels of Fig. 5 can be explained by referring back to the energy level diagram in Fig. 1. This diagram depicts the energetic situation for the dichlorobenzenes: the v'=<> level in S 1 lies less than halfway toward the ionization potential (I.P.) therefore a photon tuned to the 0-0 transition (or any transition whose energy is less than half the I.P.) cannot ionize the excited molecule via absorption of one more photon. At our laser intensities two-photon ionization from the excited state (or 1+2 REMPI) is prohibitively weak and thus no ions are created in this spectral range. The origin region of the spectrum can be observed using two-color REMPI spectroscopy (the Al + A2 scheme in Fig. 1). Such a spectrum is shown in the bottom panel of Fig. 5 in which the fourth harmonic of the Nd:YAG laser at 266 nm has been used for A2. We now can see clearly the origin transition at 279.67 nm and other red bands that were unobservable in one-color REMPI. Two-color REMPI has another advantage in that the second laser can be made intense enough to ionize essentially all of the excited molecules produced by the frrst laser. (While not shown in Fig. 5 the ion signals for the two-color spectrum are 2-3 times larger than those of the one-color spectrum.) This is possible to do with one-color REMPI, however at such high laser intensities the spectral features in the first photon resonant step become broadened by saturation and power broadening effects. In fact some of the stronger vibronic bands in the 1,4 DCB REMPI spectra shown in Fig. 5 are slightly saturated even at a pulse energy of 1 mJ. In one-color 1+1 REMPI the ion signal at low laser intensities (11) scales as 112 but when the resonant step is saturated at higher intensities the ion signal becomes closer to linear in 11. In two-color 1 + 1 REMPI the ion signal scales as 1112 at low intensities, becomes roughly independent of II and linear in 12 at high 11 and moderate 12, and finally becomes approximately independent of both 11 and 12 in the regime in which the second laser photoionizes all of the electronically excited molecules. The last situation is particularly useful because the ion signal is not sensitive to the pulse-to-pulse variations in the laser intensities and signal-to-noise is therefore improved. This. illustrates the point that, even though 1+1 REMPI is a non-linear technique, there exist intensity regimes in which it can be made almost independent of laser intensity. Finally we note that while two-color REMPI using the 266-nm laser works wonderfully in the cold molecular beam it is a disaster when used on a room temperature sample. This is because there are many hot bands near 266 nm that lead to direct 1 + 1 ionization and background ionization by the 266-nm laser alone completely swamps the resonance ionization signals due to two-color REMPI. In the molecular beam these hot bands are removed and there is almost no ionization from the 266-nm laser alone. In Fig. 6 we demonstrate the ease with which jet-cooled REMPI spectroscopy can differentiate between the 1,2 and 1,4 isomers of dichlorobenzene. This spectrum is a two-color REMPI spectrum taken on a jet-cooled sample of 1,2 DCB that contains a 1,4 DCB impurity. The reddest 1,2 DCB band at 275.92 11 |