You should send a thank you letter or note within 24 hours. The purpose is not only to thank them for their time, but to reiterate why you are the perfect candidate for the job. It can also be another opportunity to restate a point that you feel you did not make clear during the interview. A nice way to close a letter could be, “I look forward to the opportunity of being part of your company’s success.”
A true story about the power of a thank-you note: One of my clients met with 40 candidates for an opened position in her department. Not one person of the 40 candidates “sent” her a thank you note. What do you think she would have done if she had received a note or card from one of the interviewees? Perhaps take another look at that person, even if they weren’t on the top of the list? That is exactly what she said she would have done.
She did; however receive one. After an interview was over a young lady handed her an envelope. When she asked what it was, imagine her surprise when the interviewee said that it was a thank you card.
My client felt very awkward. Why? Because the purpose of a thank you card is not necessarily to thank them but to reiterate something that you spoke about in the interview or add something you may have forgotten. None of this was the case, not to mention the insincerity of the note. How could it be? It was pre-written!
A note of caution, you should only email a thank you to the interviewer if they are to make a decision that day. In this case you should state that under normal circumstances you would not send a thank you via email, but because they were going to make a decision the same day, you wanted to send them a note to follow up.