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Chapter 18: Q. 18.1 (page 451)

What is a virtual state?

Short Answer

Expert verified

The virtual state is the imaginary state that is used to couple the ground and excited states as a reference.

Step by step solution

01

Step 1. Given information

Define virtual state.

02

Step 2. Raman scattering

The following states are common in Raman scattering:

1. Ground state

2. Virtual state

3. Excited state

03

Step 2. Virtual state

The real states are the ground and excited states, whereas the virtual state is an imaginary state. As a reference, this imaginary state is used to combine the ground and excited states.

A virtual stage is depicted in the diagram below.

This includes the interaction of two photons at the same time, with the transition occurring at the same level. The energy of the molecule can be considered as any of an infinite number of values (virtual states) between the ground state and the excited state based on the frequency of radiation from the source.

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Most popular questions from this chapter

For vibrational states, the Boltzmann equation can be written as

$\frac{N_{1}}{N_{0}} = e x p (- ∆ E / k T)$

where $N_{0}$ and data-custom-editor="chemistry" $N_{1}$ are the populations of the lower and higher energy states, respectively, $∆ E$ is the energy difference between the states, $k$ is Boltzmann’s constant, and $T$ is the temperature in kelvins.

For temperatures of $20 ° C$ and data-custom-editor="chemistry" $40 ° C$ , calculate the ratios of the intensities of the anti-Stokes and Stokes lines for data-custom-editor="chemistry" ${CCl}_{4}$ at role="math" localid="1646297502302" $(a) 218 {cm}^{- 1}, (b) 459 {cm}^{- 1}, (c) 790 {cm}^{- 1}$ .

For each temperature and Raman shift, calculate how much more intense the Stokes line is compared to the anti-Stokes line.

An antihistamine shows sharp peaks at Raman shifts of $v = 488, 725, 875, 925,$ and $1350 {cm}^{- 1}$ . At what wavelengths in nanometers would the Stokes and anti-Stokes lines for the antihistamine appear if the source were

(a) a helium-neon laser (632.8 nm)?

(b) an argon-ion laser (488.0 nm)?

Assume the excitation sources in Problem 18-3 have the same power. (a) Compare the relative intensities of the Raman lines of the antihistamine for each of the two excitation sources. (b) If the intensities were recorded with a typical monochromator photomultiplier system, why would the measured intensity ratios differ from the ratio calculated in part (a)?

The following questions all deal with the similarities and differences between IR spectrometry and Raman spectrometry.

(a) What are the requirements for a vibrational mode in a molecule to show IR absorption? What are the requirements for a vibrational mode to be Raman active? Why do these requirements differ? Under what circumstances will vibrational modes by both Raman and IR be active? Under what circumstances will vibrational modes be Raman active but not IR active and vice versa?

(b) Consider the molecule chloroacetonitrile ( ${ClCH}_{2} CN$ ). How many vibrational modes should this molecule have? Why might one observe fewer Raman bands than expected?

(c) Chloroacetonitrile shows a strong Raman band at $2200 {cm}^{- 1}$ due to the C-N stretching mode. The corresponding IR absorption is very weak or absent. By comparing spectra in the $2200 {cm}^{- 1}$ region, what can you conclude about the C-N stretching mode in chloroacetonitrile?

(d) Compare and contrast IR and Raman spectrometry with respect to optics, cell materials, sample handling, solvent compatibility, and applicability to various sample types.

(e) Compare and contrast the sources and transducers used in Raman spectrometers to those used in FTIR instruments. Consider both FT-Raman and dispersive Raman spectrometers in your comparison.

(f) Compare and contrast IR and Raman spectrometry with respect to qualitative usefulness, detection limits, quantitative analysis, and instrumental complexity.

Why does the ratio of anti-Stokes to Stokes intensities increase with sample temperature?

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