electronic spectroscopy and excited states

Photon absorption creates an

Excited electronic state

An electronic transition occurs when

an electron moves from one MO to another

When an electron moves from one MO to another the electron density is

redistributed

Electronic transition is typically induced by

UV-Visible light

Chromophores

Absorbers

The sun

a natural source of photons

used by nature for photochemistry

A molecule generally occupies

the lowest energy electronic state - the ground state

Photon absorption induces

an electronic transition

What does promoting an electron to a higher energy MO create?

An excited state

When electrons occupy the same MOs but with different spin

A different excited state

Electron spin properties of a state are described by the Multiplicity

2S + 1

S

total spin

Electrons occupy

Orbitals

Molecules occupy

States

For a polyatomic molecule there are

several MOs

Atoms have

no rotational or vibrational degrees of freedom

Atoms electronic energy levels are

clearly defined

The UV-vis spectra for an atom consists of

Sharp lines

Molecules have

rotational and vibrational degrees of freedom

Molecules electron states

have many energy levels

The UV-vis for molecule consist of

many lines, or broad bands

For the born-oppenheimer approximation the electronic, vibrational and rotational properties are

considered to be independent of each other

Transitions occur from

the electronic ground state

The longest wavelength absorption arises from

the lowest energy transition to the S1 excited state

For vibrational energy transitions generally occur

• From v” = 0 in S0 state

• to several v’ levels in the S1 state

For rotational energy transitions generally occur

• from several J” rotational levels in the v” = 0 level

• To several J’ levels in each v’ level in the S1 state

Rotational fine structure seen from

gases

Vibrational structure seen from

gases, liquids, and solids

The energy of the photon required is determined by

the energy gap between ground and excited states

The probability is determined by the

selection rules

Overall selection rule for an electronic transition:

To excite an electron from one orbital to another the electric field of the light must cause a displacement of charge, giving a transition dipole moment

An electron has two possible spin states:

Up (alpha) and down (beta)

Orbital selection rule has

two parts

Symmetry - orbital part of the transition moment can be separated into components along

x, y and z axes

To be symmetry forbidden

All three components must be zero