Question

Time scale of and differences between spectroscopic processes:

a.(5 points) Rank the following in order of increasing rate (with the slowest at the top of the list): electronic absorption, fluorescence, phosphorescence, vibrational energy transfer, intersystem crossing.

b.(5 points) Explain why the rate of fluorescence is so different from that of phosphorescence. And explain important differences between the two processes.

Answer 1

Here's what I get

Step-by-step explanation:

a. Time scales

Phosphorescence 10 s to 10⁻⁴ s

Fluorescence 10⁻⁷ s to 10⁻¹⁰ s

Intersystem crossing 10⁻⁸ s to 10⁻¹⁰ s

Vibrational energy transfer 10⁻¹⁰ s to 10⁻¹² s

Absorption 10⁻¹⁵ s

The first Jablonski diagram below shows the time scales for the different processes.

b. Fluorescence vs. phosphorescence

In fluorescence, a quantum of light excites an electron to an excited singlet state (S₁).

It quickly drops back to the ground state singlet (S₀) and emits a quantum of light.

In phosphorescence. the electron moves by intersystem crossing to an excited triplet state (T₁) and then drops to S₀.

According to quantum transition rules, intersystem crossing from S₁ to T₁ and dropping from T₁ to S₀ are rather unlikely.

Thus, phosphorescence times can range from milliseconds to tens of seconds.

The second Jablonski diagram shows fluorescence and phosphorescence in more detail.