False origins

Figure 6-6a. b- anil c-polarizcd absorption speclra of single crystal at 4.2 K. O and O" arc two Herzberg-Teller false origins as discussed in the text (Section 6.4.1). The most intense vibrational modes are indicated. 

We assign the peaks al 18486 cm 1 (O ) and 18657 cm 1 (O") of Figure 6-6a to vibronic levels acting as false origins which are generated by Herzberg-Teller... 

Figure 6-8. b- and c-polaii/ed absorption and fluorescence spectra in the range betweeu 18000 cm" and 19000 cm 1. The fluorescence peaks A and C correspond lo tile false origins O and O" identified in absorption. 

Figure 6-18. T(, single irystalpolan/ed fluorescence spectra al 4.2 K. The main vibronic bands built On the origin and on the A, B, and C false origins arc indicated. See text for discussion (Section 6.6.1).

In the denominator of Eq. (29) one can detect the false origin in the spectrum at the frequency Qa — o/p+ 5a which, together with tta quanta of the accepting mode , compensates the energy of incident light. The intensity maximum is at... 

The occurrence of the intensity maximum in the Si < S0 excitation spectrum at about 12,000 cm-1 above the false origin A 1A2(41) is consistent with this assignment for two reasons. First, the 1B2 1A2 coupling which induces the... 

With temperature increase to 4.2 K, a completely different situation develops. Now, the emission stems dominantly from the substates n/ni, which carry much higher allowedness with respect to the purely electronic transition to the ground state at 21,461 cm As consequence, the vibrational modes, which are active in the emission process, are different from those found in the emission of substate I at T=1.2 K. In particular, for several fundamentals such as the 1,056 and 1,497 cm-1 modes, one can also observe weak second members of progressions (not displayed in Fig. 6). Therefore, it can be concluded that these modes represent totally symmetric Franck-Condon (FC) active modes [63, 90, 94-97], The assignment concerning FC activity is in accordance with the observation that many of the FC modes (e.g., 530, 743,1,056 cm-1) are also built upon the false origins and occur as combinations in the 1.2 K spectrum. Using the equation (see for example [49, 63, 95-98])... 

In common with other aza-naphthalenes, 1,6-naphthyridine displays a progression in the angle-bending mode. Along the a axis this progression displays a normal Franck-Condon envelope but when based on the false origins 347 and 484/8 polarized parallel to the b and c axes, this is certainly not the case. 

It should be pointed out that once the zf origin of the different bands is determined (in particular the true and false origins of the spectrum), a complete description could be given for the spin-orbit perturbations that give the lowest triplet state its radiative properties. A very extensive work was conducted by Tinti and El-Sayed (39) in which different perturbations—e.g., heating, applying a magnetic field, as well as saturation of the zf transitions with microwave radiation-were used to determine the property of the individual zf levels. The effect of these perturbations not only on the phosphorescence spectrum but also on the observed decays and polarizations has been examined. Limits on the importance of the different spin-orbit interactions are obtained. This spectroscopic work represents the type of experiments that can be done and the kind of information that can be obtained from PMDR and other methods. 

The low-temperature emission spectrum (Fig. 13) shows besides the false origins (e.g. fundamentals of 213,266,376,531,713,1293,1462 cm etc., compare Table 7) also combinations, i.e. other fundamentals are built upon these false origins (e.g. (531 -I- 383), (531 -1- 718), (531 -1- 1168), (713 -1- 718), (713 -l-1400) cm, etc.). These other vibrational modes are assigned below to totally symmetric Franck-Condon (FC) modes. 

The 20 K emission spectrum shown in Fig. 14 exhibits vibrational satellites due to activity of fundamentals (e.g. 190, 383, 458, 718, 1400,1484 cm", etc.), of combinations of these fundamentals (e.g. (190 -l- 1484), (383 -l- 1400), (458 -l-1484), (718 + 1400), (718 -I- 1484) cm", etc.), and for the most intense satellites, one observes also the second members of progressions (e.g. 1 x 718 cm, 2 X 718 cm 1 X 1484 cm, 2 X 1484 cm ). These results can be rationalized well, when all vibrational satellites with significant intensity are assigned to correspond to totally symmetric fundamentals. This assignment is also in accordance with the observation that the same fundamentals are built upon the false origins occurring in the 1.3 K emission spectrum. (Fig. 13) An assignment to an alternative symmetry would not allow us to explain the very distinct differences of vibrational activities found in the emission of the states I and II, respectively. 

The delayed emission spectrum is very similar to the time-integrated emission spectrum measured at T = 1.3 K. (Compare the Figs. 22b to 13.) Therefore, this delayed spectrum is similarly assignable as described in Sects. 4.2.1 and 4.2.4. In particular, all vibrational satellites that are marked in Fig. 22 b represent false origins. Their intensities are vibronically (Herzberg-Teller) induced. Also this delayed emission spectrum does not reveal any intensity at the position of the electronic origin I. °... 

Fig. 25. Emission spectra (time-integrated) of (a) Pt(2-thpy-hg)2 and (b) Pt(2-thpy-dg)2 dissolved in n-octane (Shpol skii matrix) at a concentration of 10 mol/1. Ag c = 457.9 nm. The wavenumber scale gives the separation from the respective electronic origin I (set to zero). For both compounds, origin I does not carry any emission intensity. All vibrational satellites that correspond to fundamentals are false origins, i. e. they are vibronically (Herzberg-Teller) induced. Several vibrational modes of the perprotonated compound are correlated to those of the perdeuterated one, they are connected by dotted lines. The spectrum (a) corresponds to the one reproduced in Fig. 13. (Compare Ref. [23])...

The mechanisms that induce the intensities of the vibrational satellites in the 1.3 K emission of state I are assigned to be the same as discussed in Sect. 4.2.4.1 for the perprotonated compound, i. e. all vibrational satelfites that correspond to fundamentals represent false origins, they are Herzberg-Teller induced. (Compare Sect. 4.2.2.)... 

The directional properties of the naphthalene absorption system pointed to interpenetrating allowed and vibrationally induced bands, the former long-axis and the latter short-axis polarized. There did, however, remain a difficulty. Given the different polarizations the separation between the true and false origin(s) must be the frequency of a nontotally symmetrical vibration. The observed hot band separation however was close to a ground state Raman-active 512 cm-1 vibration, known with certainty to be totally symmetrical. 

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