Chapter 15 Instructor’s Manual
15-1. In a fluorescence emission spectrum, the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength. In an excitation spectrum, the emission is measured at one wavelength while the excitation wavelengths are scanned. The excitation spectrum closely resembles an absorption spectrum since the emission intensity is usually proportional to the absorbance of the molecule. 15-2. (a) Fluorescence is the process in which a molecule, excited by the absorption of
radiation, emits a photon while undergoing a transition from an excited singlet electronic state to a lower state of the same spin multiplicity (e.g., a singlet → singlet transition). (b) Phosphorescence is the process in which a molecule, excited by the absorption of
radiation, emits a photon while undergoing a transition from an excited triplet state to a lower state of a different spin multiplicity (e.g., a triplet → singlet transition). (c) Resonance fluorescence is observed when an excited species emits radiation of
the same frequency at used to cause the excitation. (d) A singlet state is one in which the spins of the electrons of an atom or molecule
are all paired so there is no net spin angular momentum (e) A triplet state is one in which the spins of the electrons of an atom or molecule
are unpaired so that their spin angular moments add to give a net non-zero moment. (f) Vibrational relaxation is the process by which a molecule loses its excess
vibrational energy without emitting radiation.
Principles of Instrumental Analysis, 6th ed. (g)
Internal conversion is the intermolecular process in which a molecule crosses to a
lower electronic state with emitting radiation. (h) External conversion is a radiationless process in which a molecule loses
electronic energy while transferring that energy to the solvent or another solute. (i) Intersystem crossing is the process in which a molecule in one spin state changes
to another spin state with nearly the same total energy (e.g., singlet → triplet). (j) Predissociation occurs when a molecule changes from a higher electronic state to
an upper vibrational level of a lower electronic state in which the vibrational energy is great enough to rupture the bond. (k) Dissociation occurs when radiation promotes a molecule directly to a state with
sufficient vibrational energy for a bond to break. (l) Quantum yield is the fraction of excited molecules undergoing the process of
interest. For example, the quantum yield of fluorescence is the fraction of molecules which have absorbed radiation that fluoresce. (m) Chemiluminescence is a process by which radiation is produced as a result of a
chemical reaction. 15-3. For spectrofluorometry, the analytical signal F is proportional to the source intensity P0 and the transducer sensitivity. In spectrophotometry, the absorbance A is proportional to the ratio of P0 to P. Increasing P0 or the transducer sensitivity to P0 produces a corresponding increase in P or the sensitivity to P. Thus the ratio does not change. As a result, the sensitivity of fluorescence can be increased by increasing P0 or transducer sensitivity, but the that of absorbance does not change.
Principles of Instrumental Analysis, 6th ed.
15-4. (a) Fluorescein because of its greater structural rigidity due to the bridging –O– groups. (b) o,o’-Dihdroxyazobenzene because the –N=N– group provides rigidity that is absent in the –NH–NH– group. 15-5. Compounds that fluoresce have structures that slow the rate of nonradiative relaxation to the point where there is time for fluorescence to occur. Compounds that do not fluoresce have structures that permit rapid relaxation by nonradiative processes. 15-6. The triplet state has a long lifetime and is very susceptible to...