Resonance frequency single molecule spectroscopy

One key aspect of ENDOR spectroscopy is the nuclear relaxation time, which is generally governed by the dipolar coupling between nucleus and electron. Another key aspect is the ENDOR enhancement factor, as discussed by Geschwind [294]. The radiofrequency frequency field as experienced by the nucleus is enhanced by the ratio of the nuclear hyperfine field to the nuclear Zeeman interaction. Still another point is the selection of orientation concept introduced by Rist and Hyde [276]. In ENDOR of unordered solids, the ESR resonance condition selects molecules in a particular orientation, leading to single crystal type ENDOR. Triple resonance is also possible, irradiating simultaneously two nuclear transitions, as shown by Mobius et al. [295]. 

Vongtragool S (2004) Frequency-domain magnetic resonance spectroscopy on the mnl2-acetate single-molecule magnet. PhD Thesis, Universitat Stuttgart... 

These requirements lead to the conclusion that AFM cantilevers should be small. Only short and thin cantilevers are soft, have a high sensitivity, and a high resonance frequency. Typical cantilevers are few 100 tm long, some 10 pm wide, and 0.5-3 pm thick. Resonance frequencies are in the range of 10-400 kHz in air. Especially in the field of single-molecule force spectroscopy, several researchers aim to make even smaller cantilevers with higher resonance frequency [211, 212]. The smallest cantilevers were 10 pm long, 0.1-0.3 pm thick, and 3-5 pm wide. Further size reduction would, however, make it more and more difficult to fabricate cantilevers and to focus the laser beam onto such small structures. 

In this paper we will present some examples of the application of resonance Raman spectroscopy to the study of transition metal diatomics. The application of Raman spectroscopy to matrix-isolated metal clusters was first reported by Schulze et al. (] ). Having observed only a single line in the Raman spectrum of Ag3, Schulze concluded that the molecule was linear since a bent triatomic and an equilateral triangular geometry would have, in principle, 3 and 2 Raman-active modes. The evidence, however, is not conclusive since many Czy molecules have very weak asymmetric stretches in the Raman ( ) (for example, the V3 mode of O3 is undetectable in the Raman (3 )). Moreover, the bend (V2) of Ag3 is expected to be a very low-frequency mode, perhaps lower than one can feasibly detect in a matrix Raman experiment. 

Questions of linkage are posed and answered by asking the molecule to satisfy successively two resonance conditions. Schemes which accomplish this include Dispersed Fluorescence Spectroscopy (DF, Section 1.2.2.2 a laser is tuned to excite a single line and the spectrum of the resulting molecular fluorescence is recorded), Modulated Population Spectroscopy (MPS, Section 1.2.2.3) an intense, fixed frequency, amplitude modulated PUMP laser is used to modulate the population in the upper and lower levels connected by the laser excited transition the modulation is then detected by a frequency scanned PROBE laser), which is an example of Optical Optical Double Resonance (OODR, Section 1.2.2.3). 

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