The Company recorded stock-based compensation expense as follows in the unaudited condensed consolidated statements of income and comprehensive income (dollars in millions):

In addition to the awards above, the Company recorded stock-based compensation expense within other charges, net of $0.3 million and $0.6 million in the three months ended September 30, 2020 and 2019, respectively, and $1.0 million and $1.4 million in the nine months ended September 30, 2020 and 2019, respectively, related to the 2018 acquisition of Mestrelab Research, S.L. (Mestrelab).

At September 30, 2020, the Company expects to recognize pre-tax stock-based compensation expense of $2.7 million associated with outstanding stock option awards granted under the Company's stock plans over the weighted average remaining service period of 1.9 years. The Company also expects to recognize additional pre-tax stock-based compensation expense of $25.3 million associated with outstanding restricted stock units granted under the Bruker Corporation 2016 Incentive Compensation Plan over the weighted average remaining service period of 2.6 years.

Stock-based compensation expense is recognized on a straight-line basis over the underlying requisite service period of the stock-based award.

Stock options to purchase the Company's common stock are periodically awarded to executive officers and other employees of the Company subject to a vesting period of three to four years. The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model. Assumptions regarding volatility, expected life, dividend yield and risk-free interest rates are required for the Black-Scholes model and are presented in the table below:

(a)	In addition to the options that are vested at September 30, 2020, the Company expects a portion of the unvested options to vest in the future. Options expected to vest in the future are determined by applying an estimated forfeiture rate to the options that are unvested as of September 30, 2020.

(b)	The aggregate intrinsic value is based on the positive difference between the fair value of the Company’s common stock price on September 30, 2020 and the exercise price of the underlying stock options.

The total intrinsic value of options exercised was $2.2 million and $10.3 million for the nine months ended September 30, 2020 and 2019, respectively.

Restricted stock unit activity was as follows:


		Weighted
		Average Grant
	Shares Subject	Date
	to Restriction	Value
Outstanding at December 31, 2019	865,101	$	34.73
Granted	333,081	$	43.99
Vested	(283,048)	$	33.42
Forfeited	(48,297)	$	35.68
Outstanding at September 30, 2020	866,837	$	38.66

The total fair value of restricted stock units vested was $12.8 million and $6.6 million for the nine months ended September 30, 2020 and 2019, respectively.