2. Molecular Spectroscopy and Structure Flashcards
What are the two perpendicular components of electromagnetic radiation?
Electric field and magnetic field (these are perpendicular to each other)
B (magnetic field) and E (electric field) are in phase with each other
What are the three key properties of a wave?
Wavelength, frequency, speed
C = v.lambda
What is wavenumber?
The reciprocal of the wavelength in cm^-1
What equation allows you to find the energy of a photon?
E = hv = hc/lambda
What is the equation for the refractive index n(lambda)?
n(lambda) = c/speed of light in material
What does strength refer to in terms of waves?
Amplitude of wave - same for B and E
Which component of the electromagnetic wave dominates? By how much?
The electric field component, 10^5 times stronger than the magnetic field. All light-matter interactions thus stem from electric field interactions.
What axis is the Z axis in an electromagnetic wave?
The direction of travel
What equation represent the electric component of the electromagnetic radiation?
E(t,z) = E0.cos(omega.t - kz.Z)
E(t,z) = electric field vector at position z and time t E0 = amplitude of the wave (electric field strength) omega = angular frequency - 2pi.v t = time kz = wave vector - 2pi/lambda Z = position
What is the units of the electric field strength?
N/C or V/m
How is the energy carried by a classical light wave usually expressed?
As energy density U
What are the equations for energy density and intensity/irradiance of light waves?
Energy Density U (J/m^3) = 0.5(epsilon0)|E0|^2
Intensity I (W/m^2) = 0.5c(epsilon0)|E0|^2
Epsilon0 = permittivity of a vacuum - 8.854x10^-12 F/m E0 = amplitude of the wave c = speed of light
What was Planck’s postulate?
Light energy is quantised into discrete packets called photons with Energy hv.
What is the equation for finding the force of an electrically charged particle?
F = qE
F = force of particle q = electric charge of particle E = electric field
How does a radio antenna work?
Movement of an electron in a transmitter generates an electric field. The filed may then move through space as an EM wave, this hits a receiver and causes an electron to feel the oscillating electric field. This causes the 2nd electron to start moving (electrons raised up an energy level).
What is the ground state of an electron?
A point at which the electron is simultaneously in it’s lowest vibrational, rotational and electronic energy levels. Molecules in levels higher than this are said to be in the excited state.
Can an electron be excited differently and simultaneously?
Yes, a molecule may absorb a photon that excites it both electronically and vibrationally (as long as photons energy = electronic + vibrational excitation energy)
Can the energy of a photon be partially absorbed?
No, once a molecule absorbs a photon all the energy of that photon must be absorbed.
What equation determines the electronic states of hydrogen atoms?
Rydberg equation:
En = -h.c.R/n^2
En = energy of level n n = energy levels - 1, 2, 3 etc
What is the resonance condition?
The ability of an atom/molecule to absorb a photon ONLY when the energy of the photon corresponds precisely to the energy separation between 2 quantum states.
How may an excited molecule lower it’s energy?
- Spontaneous emission of a photon
- stimulated photon emission by absorption of a second photon (LASER)
For a system at equilibrium at room temperature, are all molecules in their ground rotational, vibrational and electronic states?
Nope, at equilibrium the molecules int he system will be spread between different energy levels depending on the amount of thermal energy available to them. Here the lower energy levels will be more populated than the higher energy levels though.
What is the boltzmann distribution equation?
ni/n0 = (gi/g0)exp(-deltaE/Kb.T)
ni = number of molecules in energy level i n0 = number of molecules in the ground state gi = degeneracy of level i g0 = degeneracy of the ground state exp = e to the power ... deltaE = difference in energy between level i and ground state Kb = boltzmann constant (1.38x10^-23 J/K) T = temperature in K
How are the four states populated at varying temperatures?
NMR: almost equally populated
Rotational: excited are well populated
Vibrational: excited state only populated at high temperatures
Electronic: will remain in ground state unless you excited them with something other than heat (although there’ll be a few in excited state as you increase temp)