Rydberg bands

The fluorescence excitation spectra closely follow the Rydberg bands, indicating that the dissociation occurs from the Rydberg states. The fluorescence yield is only I. , at 1216 A. 

The second region of absorption (110-145 nm) in 0 corresponds to the transition bIa - X A and is characterized by diffuse bands and sharp Rydberg bands which are superimposed on a continuum. In this region processes 3 and 4 have been found. 

Benzene has been more intensively studied than any other organic compound. Four transitions are known between the visible region and about 1850 A, and Rydberg bands occur in the farther ultraviolet. The main features of the transitions above 1850 A are summarized in Table 9. The Rydberg series converge to the ground state of the C6H6+ ion at 9-247 eV (Price and Wood, 1935 Wilkinson, 1956). 

The longest wavelength absorption of 112 is centered around 54 000 cm This is a very weak band (e 10) and should be associated with the Ai(3e - 4e ) excited state. In solution the absorption then rises to a medium intense band at ca. 70 000 cm and reaches an intense maximum at ca. 83 000 cm Mn the gas phase two structured bands appear at 62000 cm and 76 000 cm According to equation 21 these bands result from 3e - 3p and 3e - 4p Rydberg transitions ((5 = 0.68). As expected for a Rydberg band the 62 000 cm band disappears going from the gas phase to the condensed phase. 

Enflurane (CFjHOCFjCHFCl) contains only one Cl and it is attached to a carbon to which an H and an F atom are also attached. The first band is a shoulder at about 57500 cm", followed by another shoulder near 60000 and a peak at 64000 cm " he complexity of the spectrum is probably due to the significant split in the Cl IPs, about 0.4 eV or 3200 cm". The more intense Rydberg bands are located near 70500, 74700 and 78000 cm" ... 

Saturated hydrocarbons are generally transparent down to 170 m/x and are therefore suitable for use as solvents. The transitions in paraffins and some of their simple derivatives were discussed earlier in Chapter 3. Simple paraffins exhibit bands due to o-> a transitions (CH4> Amax 125 m/x C2H Amax 135 mu) which occur just prior to the sharp Rydberg bands caused by the ionization of the molecules. Quite often it is difficult to distinguish bands due to a -> y transitions from the ionization bands. The derivatives of the saturated hydrocarbons containing atoms with unshared pairs of electrons... 

Rydberg bands form series converging to an ionization potential related to a given state of the positive ion. 

In certain highly substituted derivatives of ethylene, like tetramethyl [117-120], or di-, tri-, and tetrafluoroethylenes [121] there is a well pronounced band at frequencies lower than that of the singlet N —> Vband. For some time the origin of these bands was unknown and in a semihumoristic manner they were called the mystery band . At present we know that these are just the (n, 3s) Rydberg bands which for such molecules pass to the low frequency side of the (n, 7t ) band. 

The far ultraviolet spectra of molecules are characterized by the coexistence of bands due to valence-shell (intravalency) and Rydberg transitions. Chemists who received their training in electronic spectroscopy in connection with the tr electron systems of aromatic molecules are usuaUy surprised when they are told about molecular Rydberg transitions. Yet, beyond 200 nm and armetimes even at loiter. wavelengths the existence of Rydberg bands is a fact of life. No spectrum can be interpreted without them. 

For atoms Rydbeig tranations are simply those in which the principal quantum number (n) increases. Since the size of atomic orbitals rapidly increases with n the orbital of the excited electron is of a much larger size than those of the electrons remaining in the core. Rydberg bands can be ordered into series converging to an ionization potential related to a given state of the positive ion ... 

Still other Rydberg bands can originate with the a2 MO of the ground state. 

The IP is needed to assign the bands together with quantum defects or term values. Rydberg bands follow the IP. With the knowledge of the ranges for ns, np, nd,.. 

In butadiene and other diolefins valence-shell (N,V) and Rydberg bands still coincide near the onset of absorption -... 

In all aromatic molecules the valence-shell (rr, it ) bands are the bands of lowest energy. We usually encounter two or three of them before reaching the lowest Rydberg band. 

Teller split Eg state interacts with the other Ag (3aig) state of the ion causing the complicated band structure of the PE bands at 12.7 and 13.4 eV. This strongly suggests that the initiating orbital of the structured 3p Rydberg band is also the leg orbital. Furthermore, the photochemistry of ethane indicates that most (but not aU) of Its photolysis takes place at the (ICg, 3s) state. 

The UV spectrum of cyclopropane is quite different from those of the alkanes we have examined so far. Whereas acyclic alkanes and cyclic alkanes other than cyclopropane and cyclobutane have only Rydberg bands in the lower frequency (and photo chemically important) part of the spectrum, for cyclopropane the lowest Rydberg bands intermingle with bands due to valence-shell transitions. Two structured bands at 63000 and 78000 cm (159 and 128 nm) have been assigned to the 3e a,3p) and (3e o,4p) transitions on the grounds of the similarity of their vibrational fine structure with that of the respective photoelectron bands and their term values. Two other bands, near 70000 cm (143 nm) and 83000 cm (120 nm)... 

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