Well, I don't see why you need to create a dictionary. You could just generate the letter combinations as the program runs. No need to generate all the combinations ahead of time.
As for generating every combination, your alphabet has 62 characters in it. So, the number of combinations up to 8 characters in length, having at least one character is:
62 + 62^2 + 62^3 + 62^4 + 62^5 + 62^6+ 62^7 + 62^8 = 221,919,451,578,090
That is about 222 trillion combinations. So, you would need to buy about 200 terabyte hard drives to store your dictionary. Are you sure you want to do that?
Also, if you could generate and write to disk 1 billion combinations per second, then it would take 479 years to write all the combinations, so you would have to have your descendants continue the project after you had passed on.